Investors have convinced themselves that the credit crisis will push economies worldwide into the deepest recession since the Great Depression.

But since the Great Depression, governments have become far more aggressive about intervening when credit markets seize up or economies struggle. And those interventions have generally succeeded.

The recessions since World War II, while hardly easy, have been far less painful than the Great Depression.

Now some veteran investors, including G. Kenneth Heebner, a mutual fund manager who has one of the best long-term track records on Wall Street, say that the sell-off has gone much too far and stocks are poised to rally powerfully if the downturn is less severe than investors fear.

''The fact is, there are a lot of tremendous bargains out there,'' said Heebner, who manages about $10 billion in several mutual funds. Indeed, by many measures, stocks are as cheap as they have been in the last 25 years.

Heebner said the market plunge in the last week was no longer being driven by rational analysis. Stocks are probably falling because of a combination of panic and forced selling by hedge funds that must meet margin calls from their lenders, he said.

He said investors with a stomach for risk and a long time horizon should consider following Warren Buffett, who in the last three weeks has invested $8 billion in Goldman Sachs and General Electric.

Heebner is not alone in his optimism.

''I think in years to come — I wouldn't say months to come — we will perceive this as being a great value-buying opportunity,'' said David P. Stowell, a finance professor at Northwestern and a former managing director at JPMorgan Chase. ''Two and three years from now, it will seem very smart.''

Even before their jaw-dropping plunge of the last month, stocks were not expensive by historical standards, based on fundamentals like earnings and cash flow.

Now, after falling 30 percent or more since early September, stocks in stalwart, profitable corporations like Nokia, Exxon Mobil and Boeing are trading at nine times their annual profits per share or less.

Many smaller companies are even cheaper.

Stephen Haber, an economic historian and senior fellow at the Hoover Institution, said he believed that fear might have gone too far.

''If there is good and wise policy, and government moves effectively, this need not play itself out in ways like the Great Depression,'' Haber said.