A sudden reversal in United States stock prices late in trading on Tuesday produced a sixth consecutive session of losses and heightened uncertainty about the challenges facing global markets.

The wild swings in prices over the last two days have been the most extreme since the financial crisis. The benchmark Standard & Poor’s 500-stock index surged as much as 2.9 percent on Tuesday, but ended down 1.4 percent.

The resurgence of volatility has overturned a sense of comfort among many investors who had grown accustomed to calm markets and steadily rising stock prices in recent years.

While the market turmoil may not yet be flashing warning signs about the United States economy, which still appears to be strengthening, it is pushing investors to take a closer look at their portfolios. Stock prices have generally been rising faster than the profits of corporate America, and that is prompting caution. Even after the recent downturn, investors are still paying more for corporate profits than they have on average over the last 10 years.