King Fahd of Saudi Arabia has agreed to come here next month for the first time in eight years to discuss the Middle East situation, and Egyptian President Hosni Mubarak will pay a similar call on President Reagan in March, U.S. officials and diplomatic sources said yesterday.

The sources said the visits, while arranged separately, stem from concern among moderate Arab leaders that the United States is not taking a sufficiently active role in trying to resolve such perennial Mideast problems as the Palestinian question and the dangers the Iran-Iraq war poses to the Persian Gulf.

On the U.S. side, the sources added, the administration will seek to assure Fahd and Mubarak that its current emphasis on launching new arms control talks with the Soviet Union does not mean a downgrading of American interest in the Mideast. Instead, the sources said, the United States views the visits as an opportunity to discuss such issues as long-pending requests from moderate Arab states to buy large quantities of U.S. weaponry and to explore possible new initiatives for restarting an Arab-Israeli dialogue on the Palestinian issue.

In that connection, the sources said, the visit by Fahd, expected early next month, is especially significant because he has not been here since the spring of 1977, before the Camp David summit that produced the Egyptian-Israeli peace agreement.

Saudi Arabia's opposition to the Camp David process caused Fahd to cancel visits with President Jimmy Carter in 1979 and with Reagan in 1981; until now, the king has fended off all U.S. efforts to entice him into coming here. He did meet with Reagan at the Cancun summit in Mexico in October 1981.

The diplomatic sources said that Fahd's decision to depart from his cautious policy of not calling too much attention to Saudi Arabia's relations with the United States reflects a growing concern that the administration, having suffered a major defeat last year in Lebanon, has adopted a mark-time attitude toward the Middle East, permitting the radical states of the region such as Syria and Iran to gain influence at the expense of the moderates.

The sources said that Faud and Mubarak are expected to renew Arab calls for the United States to open a dialogue with Palestine Liberation Organization Chairman Yasser Arafat and to put pressure on Israel to agree to an international conference, involving the PLO and the Soviet Union, to deal with Palestinian self-determination and Israeli withdrawal from Arab territories.

Mubarak, who is seeking to end Egypt's isolation from the Arab bloc and reestablish his country as a leading force in the region, has been especially vocal in arguing that such a U.S. move is necessary if the balance of Mideast power is to tip toward the pro-western moderate states rather than the Arab radicals. His efforts to reestablish a pro-western alliance composed of Egypt, Saudi Arabia, Jordan, Iraq and the moderate, Arafat-led faction of the PLO have been encouraged by Fahd.

However, U.S. sources say there is no chance that the administration will abandon its commitment to Israel not to recognize the PLO or will try to force Israel into an international conference. Instead, the U.S. view is that Israel, struggling to revive its inflation-ravaged economy, is currently too preoccupied to handle major new efforts at resolving the Arab-Israeli conflict and that emphasis should be on more modest goals, such as finding ways for Israel to withdraw from southern Lebanon and to begin a dialogue with Jordan's King Hussein.

The talks with Fahd are also expected to include discussion of Saudi Arabia's desire to buy millions of dollars' worth of U.S.-made tanks, jet fighters, air transports and various ground- and air-launched missiles. The administration, which delayed acting on these requests during last year's election campaign, is expected to approve most of them to help safeguard the gulf's oil supplies from possible expansionist moves by Iran.

Mubarak, who is expected here the second week of March, will be discussing U.S. aid for the coming year. During fiscal 1985, Egypt -- the second-largest recipient of U.S. assistance, after Israel -- is scheduled to receive $1.2 billion in military aid and $815 million in economic aid, all in the form of grants that do not have to be repaid.

Egyptian Foreign Minister Esmat Abdel-Meguid is expected here next month to make preparations for Mubarak's trip.