Lawmakers in Washington were due to vote on their latest effort to break an impasse on funding the government operations and so far investors did not appear unduly concerned the deadlock would drag on.
Meanwhile, marquee corporate names were expected to report quarterly earnings during the week, including General Electric, which has been floundering, as well as Netflix, Caterpillar and American Airlines.
Ten minutes into the day's trading, the blue-chip Dow Jones Industrial Average had fallen 0.2 percent to 26,013.45, while the broader S&P 500 and tech-heavy Nasdaq were both essentially flat at 2,808.84 and 7,335.55 respectively.
Patrick O'Hare of Briefing.com said Day Three of a government shutdown was not enough to worry markets excessively.
"The game of political brinkmanship has been played so many times now that market participants aren't going to be unnerved too greatly at this stage of the game," said Patrick O'Hare of Briefing.com.
Insurance giant AIG fell 0.9 percent after revealing a plan to buy Validus Holdings for $5.6 billion, or $68 per share. Validus, meanwhile, soared more than 44 percent.
Biotech company Celgene was 1.2 percent lower after announcing it would buy bio-pharmaceutical firm Juno Therapeutics for $9 billion, or $87 per share. Juno jumped 27 percent on the news.
Drugmaker Sanofi fell 4.2 percent on plans to acquire the biotech firm Bioverativ for $11.6 billion, which jumped 62.6 percent.
Meanwhile, the International Money Fund boosted its forecasts for global growth this year and next, citing an initial increase from last month's Republican tax cuts, but warned that overheated financial markets, which have rallied for over a year, could be due for a reversal.