The Rev. Lawrence Jenco, a Roman Catholic priest held hostage by Shiite Moslem militants in Lebanon for 18 months, was released unharmed today and said, after his jubilant arrival here, that he had been with three other American captives and they were in good health.

Jenco, who was found on a road in the Syrian-controlled southern Bekaa Valley early today, was turned over to U.S. Ambassador William Eagleton here after talks with Syrian authorities and an emotional meeting with the sister of one of the Americans still being held hostage. American officials said he would probably be flown to a U.S. military base in Frankfurt, West Germany, on Sunday.

Speaking to reporters, Jenco, 51, who was seized by gunmen in Moslem west Beirut on Jan. 8, 1985, thanked the Syrians "for their efforts in securing my release," and added, "I'm sure they will keep up their efforts to secure the release of the remaining hostages."

He gave no details about Syrian efforts, but his release was seen in Beirut as a result of a three-week-old Syrian Army move to impose order in the strife-torn Lebanese capital, special correspondent Nora Boustany reported from there.

In Washington, President Reagan said he was "grateful that our prayers for the safe return of Jenco have been answered" and he thanked the Syrian government for the way it "expedited the delivery" of the priest to Damascus.

Reagan added that "while we continue to work for the safe release of all those held, we hold their captors and those who support them responsible for the safety of the remaining American hostages."

David Jacobsen, one of the remaining hostages, appealed to the American public in a videotape brought out by Jenco to pressure the Reagan administration to negotiate the release of the other hostages, The Associated Press reported.

Vice President Bush, in Frankfurt on his way to a previously planned visit in the Middle East, spoke with Jenco by telephone and described him as "quiet, subdued," but "in pretty good health."

Islamic Jihad, the pro-Iranian Shiite terrorist group that has said it is holding the American hostages, had said in a statement published in Beirut last night that it was releasing Jenco as a "good-will gesture" because of his "deteriorating health" and it expressed its "animosity" toward the Reagan administration, warning that from now on the U.S. government would "bear the full responsibility for the lives of the rest of the hostages."

U.S. officials announced tonight that Robert Oakley, head of the State Department's Office for Counterterrorism and Emergency Planning, was flying to Damascus to accompany Jenco home.

The officials said it was too soon to tell whether Jenco's release suggested any movement toward the freeing of the other hostages. They noted, however, that the stated reason for his release was his health, and they said they had no indication anything further was involved.

Jenco, who headed the Beirut office of Catholic Relief Services, the New York-based Roman Catholic aid program, gave reporters no details of his release. He was found near the Qaroun Dam, 50 miles southeast of Beirut, in the southern Bekaa Valley, taken to a nearby Syrian military intelligence office and then brought to Damascus.

As he emerged from the car on his arrival at the Foreign Ministry here, he asked immediately for Peggy Say, who is here working for the release of her brother, Associated Press correspondent Terry Anderson, one of the other American hostages.

Jenco and Say embraced warmly, crying and kissing as journalists thronged around and Syrian security guards rushed them into the ministry for a 45-minute meeting with Foreign Minister Farouk Sharaa, other Syrian officials and Eagleton.

Jenco appeared in good health and spirits as he spoke briefly to reporters there and told them, "I'm just so happy to be out and on my way home." But he gave few details of his captivity, saying, "I need to keep quiet now for a while. . . . I need space to kind of unwind."

He said he had been able to see the three other American hostages known to be alive -- Anderson; Jacobsen, administrator of the American University Hospital in Beirut; and Thomas Sutherland, dean of the university's school of agriculture. He said they were in good condition, but he gave no further details.

Jenco said nothing about William Buckley, a U.S. Embassy political officer kidnaped in March 1984. Islamic Jihad claimed last October that it had killed Buckley, but his body has never been found. The last American hostage to be freed was the Rev. Benjamin Weir, a Presbyterian minister, released Sept. 15.

At the U.S. ambassador's residence, Jenco was visited by the papal nuncio here, who said mass with him at Jenco's request, Marjorie Ransom, a spokeswoman for the embassy said.

Say said that when they spoke, Jenco "seemed to radiate joy and tears, absolute ecstacy at being free."

Asked whether Jenco's release had heightened her hopes for her brother, Say said: "Every time a hostage has come out, there's hope for the rest of us . . . that we can reach out and touch the rest of them."

Foreign Minister Sharaa, with whom Jenco shook hands warmly, said President Hafez Assad had asked him to convey to Jenco "his best greetings and congratulations" on his release. Sharaa said Syria had "exerted a lot of efforts in the past" for the release of the hostages. "We are going to continue these efforts," he said.

Boustany reported from Beirut:

Jenco is the first American hostage freed in 10 months and the first westerner freed since Syrian troops deployed in west Beirut three weeks ago. Agreement by Hezbollah -- a militant Moslem fundamentalist group believed linked to Islamic Jihad -- to expand the Syrian security plan into Beirut's Shiite southern suburbs preceded Jenco's release.

Syrian officials reportedly have told Say that the newly implemented security measures in Lebanon included efforts to find her brother and other hostages. Say flew to Syria several days ago and met with Defense Minister Mustafa Tlas on behalf of the hostages.

The Islamic Jihad's statement citing Jenco's "deteriorating health" was seen by some analysts as a face-saving way out for the kidnapers, who seemed to be acting under Syrian pressure.

Syria has been eager to improve its image in the West by showing concern for the hostages. Syria's military intelligence chief in Lebanon, Brig. Ghazi Kanaan, said earlier this month that appeals have been made to all groups here to set the hostages free, and Assad last month received Rep. Robert Dornan (R-Calif.), who brought an appeal from 251 members of Congress for help in gaining the hostages' release.