President Reagan, whose accessibility to the news media has been severely limited since the Iran-contra affair was disclosed last November, will hold his second domestic news conference of the year at 8 tonight, White House spokesman Marlin Fitzwater announced yesterday.

It will be the 42nd news conference of the Reagan presidency, an average of slightly more than one every two months. But 26 of these news conferences were held during Reagan's first term, and they have become especially rare since Howard H. Baker Jr. became White House chief of staff earlier this year.

Reagan has held only two news conferences since last November, one in Washington on March 19 and another in Venice on June 11 at the conclusion of the economic summit of the industrialized nations.

Other than these two events, most of Reagan's responses to questions this year have come during "photo opportunities," as the White House calls picture-taking sessions where the president sometimes answers shouted questions from reporters. Typically, these sessions have lasted two minutes or less.

Privately, White House officials have acknowledged that Reagan's unwillingness to hold news conferences during much of this year reflected concern about the Iran-contra investigations. Many of the statements made by Reagan at a news conference last Nov. 19, soon after the arms-for-hostages trade with Iran was disclosed, turned out to be misleading or inaccurate.

But White House officials' reluctance to hold news conferences continued even after conclusion of the Iran-contra hearings, partially out of concern about what Reagan might say. The president's comment at his poolside Venice news conference that "there could still be some lowering" of the U.S. dollar prompted an immediate correction by aides who were fearful that the remark would undermine the stated U.S. position favoring dollar stability.

Some aides said last week that the news conference tonight would be timed to announce the dates of a superpower summit already agreed to between Reagan and Soviet leader Mikhail Gorbachev.

But a senior official said yesterday that Soviet officials had not yet presented proposed dates to Secretary of State George P. Shultz, who is en route to Moscow. The official said there is "still a possibility, but not a probability" that Reagan would be able to announce the summit dates tonight.