Anticipation of the impeachment probe was a factor in the stock market's retreat on Tuesday.
But US stock investors appeared to be taking the news in stride after House Democratic Leader Nancy Pelosi formally announced the probe.
Wednesday's flattish early trading "suggests that the market isn't going to allow itself to get too worked about the inquiry at this stage of matters when the headlines are heavy but the tradable facts of import are light," analyst Patrick O'Hare wrote at Briefing.com.
About 20 minutes into trading, the Dow Jones Industrial Average stood at 26,841.85, up 0.1 percent.
The broad-based S&P 500 was essentially flat at 2,965.68, while the tech-rich Nasdaq Composite Index had shed 0.2 percent at 7,978.90.
Among individual companies, Dow member Nike surged 5.2 percent after reporting better-than-expected quarterly profits on strong direct sales and higher revenues from China.
Boeing also added to the Dow's gains, advancing 0.6 percent as it announced a number of reforms to its board and corporate structure to highlight safety concerns after two crashes led to 346 fatalities and the grounding of its popular 737 MAX plane.
Philip Morris International jumped 5.9 percent as it and Altria called off a potential $200 billion merger following a sudden surge in worries about vaping.
Altria, which holds a major stake in e-cigarette company Juul, gained 0.5 percent.