Seven of the 11 major S&P sectors were lower, with the energy index's 0.97 percent fall leading the decliners.
At 12:37 p.m. ET (1637 GMT), the Dow Jones Industrial Average was down 37.39 points, or 0.17 percent, at 21,429.75, the S&P 500 was down 1.1 point, or 0.05 percent, at 2,435.93.
The S&P energy index was the S&P's weakest sector with a 1.5 percent decline. Brent ended below $45 a barrel.
After being up by as much as 1.6 percent earlier in the day, benchmark USA crude turned lower and fell 98 cents, or 2.3 percent, to settle at $42.53 per barrel. The two-year yield dipped to 1.34 percent from 1.35 percent late Tuesday, and the 30-year yield fell to 2.73 percent from 2.74 percent. They are down almost 15 percent for the year, when the overall S&P 500 is up 8.8 percent. Its forecast for revenue and earnings this fiscal year also topped analysts' expectations. Its stock rose $7.59, or 8.4 percent, to $97.55.
Among stocks, Lennar rose 4.2 percent to $54.91 after the No. 2 USA homebuilder reported a higher-than-expected quarterly profit.
Both oil benchmarks - US crude and Brent - are down more than 15 percent since late May, raising concerns that prices could fall further in the near-term.
"If oil prices continue to move lower, obviously that would be bad for inflation or even result in disinflation". Shares of Intel Corp (NASDAQ: INTC) fell 1.8% and Walt Disney Co (NYSE: DIS) finished 1.4% lower.
MSCI has been considering including these A-shares in its index for years but had demurred until now due to a range of concerns, including their inaccessibility for global investors.
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Many investors expect the so-called A shares that make up the majority of China's stock market are likely to be included after being rejected on three previous occasions.
The Nasdaq Composite was down 36.55 points, or 0.59 percent, at 6,202.46.
Transocean fell 43 cents, or 5 percent, to $8.14, Halliburton dropped $1.44, or 3.3 percent, to $42.53 and National Oilwell Varco lost $1.03, or 3.1 percent, to $32.55.
TREASURY YIELDS: Bond prices were flat.
TREASURY YIELDS: The 10-year Treasury yield dipped to 2.15 percent from 2.16 percent late Tuesday.
The British pound rose to $1.2668 from $1.2629 late Tuesday.
CURRENCIES: The dollar weakened to 111.25 yen from 111.46, while the euro was almost flat at $1.113.
Parexel International was up 3.8 per cent to US$87.11 after private equity firm Pamplona Capital is nearing a deal to acquire the pharmaceutical research services provider for more than US$4.5 billion.