Now that the Olympic flame has been doused, Chinese leaders have to turn their attention from holding mass spectacles in Beijing to actually running the country. Their most urgent task is to manage their economy.

At first glance, that should not be too difficult as they have momentum on their side. According to official statistics, which appear to undercount economic performance these days, the economy grew 10.4% in the first half of the year. That’s only slightly below last year’s 11.9% increase, the biggest jump since 1994.

Yet despite the healthy numbers, the leaders in Beijing are concerned about the downward trend in growth. At its July 25 meeting, the ruling Politburo officially reversed gears, adopting stimulus policies and weakening anti-inflation efforts that were announced last November.

Concurrently, the central bank, the People’s Bank of China, dropped its policy of monetary tightening and adopted a plan designed to produce “stable, relatively fast growth.” As a result, it increased the loan quotas of commercial banks by 5% to help struggling businesses, especially those in the export and agricultural sectors.

The State Administration of Taxation boosted export tax rebates at the same time. Beijing then pushed currency traders to drive down the renminbi, which in late July suffered its largest one-day decline against the dollar since the currency was partially unpegged from the dollar in 2005.

Rumors are now circulating of a stimulus package of 400 billion yuan, about $59 billion. On Monday, the central bank reduced lending rates for the first time in four years and loosened reserve requirements for the first time in nine.

The nation’s fight against persistent inflation has hardly been won, so the Politburo’s switch to stimulus policies seems especially risky. But, Chinese leaders are right to be concerned about a stalling economy: three trends are now coinciding to produce a period of low — or no — growth.

First, China appears to be at the end of a decade-long expansionary phase. Last year was the fifth such period of double-digit growth. When we look back at recent history, the last two half-decades of such fast expansion both ended in downturns, a mild one at the end of last decade and a severe one after the Tiananmen massacre. No one, especially the current economic chief, Premier Wen Jiabao, appears able to eliminate China’s volatile cycles.

Second, the Olympics, which stimulated the economy before the Games, are bound to be a negative factor now that the tourists and athletes have gone home and the sponsorship money has been spent.

True, China’s 24.953 trillion yuan economy is big enough to withstand an Olympic hangover, but Beijing’s leaders splurged big on the Games. They admitted to $42.6 billion in expenditures, but that figure, even if it is comprehensive, does not include the incalculable costs of stopping industry, halting construction, and restricting transportation in order to ensure clean air for the extravaganza. And it does not appear to include the costs in the five co-host cities and miscellaneous “Olympic” expenditures across the country.

Third, China’s export-led economy faces weakening consumer sentiment across the globe — and especially in America. Last year, sales to America accounted for all but $5.9 billion of the country’s overall trade surplus of $262.2 billion. A weak American economy has already been a factor in this year’s closure of 10,000 factories in China’s export powerhouse, the Pearl River Delta in Guangdong province. Analysts expect another 20,000 in that area will go out of business by December. Declining prospects for the export sector is probably the primary reason for the Politburo’s about-face on growth in late July.

Due to the convergence of unfavorable factors, it seems that just about everyone in China expects a slowdown. Beijing academics have been talking about a 2009 recession for at least three years now.

Today, the feeling appears to be shared widely in China. When I was there in June and July, I noticed that people along the country’s prosperous coast expected the economy to sour soon. For example, a well-known fund manager in Beijing told me in late June that, although he was not selling assets himself, all his acquaintances were trying to unload their holdings before the Olympics, and even he thought the coming downturn would be “very bad.” When a man who has a direct stake in selling optimism talks in darkish tones, it’s clear that recessionary psychology has taken hold.

Chinese leaders have been able to defy grim predictions — their own and others’ — for a long time. For a decade, they have kept growing by pumping central government cash into the economy, especially into their lagging state sectors.

Yet China is becoming a nation of underutilized roads, empty hotels, and excess inventory. No amount of additional spending can overcome the impediments to growth caused by the central government’s burdensome rules, such as price controls, that restrict production.

China is getting to the point where market liberalization is becoming a prerequisite for sustainable growth. Unfortunately, Beijing’s technocrats, obsessed with retaining control, will not get out of the way.

Mr. Chang is the author of “The Coming Collapse of China.”