The two sides are discussing a deal in which NBC Universal (search) would take over Microsoft's stake in the venture, while Microsoft would maintain access to some of NBC's content for use over the Internet, sources told The Post.
MSNBC was founded in 1996, and Microsoft made a $250 million initial investment.
Even if NBC takes full control of the network, the two sides will maintain a content partnership for many years, sources said.
Microsoft is also a partner in MSNBC.com, which feeds in to Microsoft's MSN site.
Spokespersons for NBC and Microsoft declined comment.
For Microsoft, selling its stake is in line with its strategy of shedding its media and Internet-related business and focusing on its core software and computer businees. The company recently sold the online magazine Slate to The Washington Post (search) and previously sold the online travel site Expedia.
"I think Microsoft is looking at where their priorities are," said a source. "They made big bets on media."
General Electric's (GE) NBC Universal, meanwhile, would prefer to have full ownership of its assets, especially given that part of GE's long-term planning is a possible spin-off of NBC Universal into a separate publicly-traded company, a source said.
GE owns 80 percent of NBC Universal, with the French conglomerate Vivendi (V) owning 20 percent.
Last year GE and Vivendi completed the merger of NBC with Vivendi Universal, a deal that combined Universal Studios, a television production unit and several cable networks with NBC.
In 2006 both sides will retain an investment bank to appraise the company, and Vivendi will then be able to begin cashing out of its stake.
MSNBC lags both Fox News Channel and CNN in the ratings. In February, for example, MSNBC saw its ratings decline 15 percent overall and 14 percent in primetime, according to Nielsen Media Research.
Among the shows at MSNBC that posted big ratings drops last month were "Hardball," which dropped 20 percent; "Lester Holt Live," which fell 39 percent; "Abrams Report," which dropped 28 percent; and "Countdown," whose ratings fell 22 percent.
Last year Jeff Zucker, NBC Universal's president of the entertainment, news and cable group, tapped networks news veteran Rick Kaplan to take the helm at MSNB. He replaced Erik Sorensen, who had run MSNBC since 1998.
NBC's other cable news network, CNBC, is also in turmoil and posting sharp ratings declines. The network, which is still wildly profitable for NBC, recently overhauled its top management.
Pamela Thomas-Graham, who had been president and CEO of CNBC since 2001, was stripped of her two operating titles and given the title of chairman. Brought in to run the network on a day-to-day basis was Mark Hoffman, who became president.
Fox News Channel, owned by News Corp., operates FOXNews.com.