United States stocks reversed earlier gains that had taken the Dow and S&P to fresh record highs, absorbing late-session losses that brought US markets into negative territory.
Earlier in the session, the Dow Jones Industrial Average topped 16,000 for the first time, extending a rally fuelled by the combination of steady economic growth and central-bank stimulus.
The blue-chip index rose as much as 0.4% to 16,030 shortly after the open, but wobbled around the 16,000 mark later in the session, only to fall back below 16,000 in the session's final hour.
The Dow was trading down 8.10 points, or 0.05%, to 15,953.60 points.
The Dow has risen about 22% since the start of the year and 144% from its post-financial crisis lows.
The S&P 500 index also retreated from record highs, sliding 8.90 points, or 0.49%, to 1,789.28 points, easing from an early gain that sent the index above 1,800 for the first time.
The Nasdaq Composite Index fell 41.55 points, or 1.04%, to 3,944.42 points.
The speed of the rally has some investors taken aback, but many money managers are coming around to the idea that stocks can stay aloft even if the Federal Reserve starts to rein in its easy-money policies. It took the Dow 136 trading days to notch its latest 1,000-point gain. That is the sixth-fastest rally of this magnitude in history.
Despite the late-session losses, experts remain mostly upbeat. While corporate earnings have been tepid of late and stocks are getting pricier, many investors believe that stock valuations haven't become too stretched — at least not yet.
"The market has continued to push forward, in part because we're looking ahead and believing economic growth is looking pretty decent next year," said Cam Albright, director of asset allocation at Wilmington Trust Investment Advisors.
"Earnings growth, while probably not spectacular, is looking to be pretty reasonable."
The S&P 500 is on pace to show earnings growth of 3.6% in the third quarter versus last year, according to FactSet. That is up from growth of 2.6% in the second quarter and a 0.4% decline in corporate profits in the third quarter of 2012.
Mr Albright said his firm still has an "overweight" position on the US stock market, saying returns in stocks are likely to remain better than those in other assets, like fixed income.
US stock investors initially took their cue from overseas markets. In Asia, Chinese stocks rallied after the Chinese government announced a broad outlook for economic reform, including opening the financial sector and relaxing restrictions on investment. The government also said it was looking to improve the country's initial public offering system.
Optimism about growth in China filtered through to European markets as well. Germany's central bank said in a monthly report that there is a good chance the momentum of Germany's economy, the engine for growth in Europe, will accelerate in the coming months.
But many investors acknowledged that stocks are rising in an atmosphere of trepidation. Monday's intraday gains weren't particularly big, and volume was light, traders said.
"Personally, I'm getting a little concerned about where we're at," said Dean Junkans, chief investment officer for Wells Fargo Private Bank.
"It feels a little bit like 2007 in some respects where market participants became quite complacent about risk."
Among noteworthy stock movers, Dow component Boeing gained after the company said it received 259 orders and commitments for its new 777X jetliner, with a list value of $US95 billion. Boeing claimed it was the largest product launch in commercial-jetliner history.
Fellow Dow member JPMorgan Chase rose after saying late Friday that it reached an agreement to pay $US4.5 billion to investors seeking to recover losses from mortgage-backed securities sold before the financial crisis.