President Carter said today that despite recent setbacks he remains optimistic that a strategic arms limitation treaty agreement and a Middle East peace treaty will be achieved.

"I think we will have a peace treaty in the Middle East, and I think we will have a SALT agreement with the Soviets. It just takes time. The complications of all the issues are not casily resolved. But I still feel hopeful, and I and Secretary (of State Cyrus R.) Vance both agree that we are not discouraged," the president said.

Carter, who is in Plains for a family Christmas, chatted with reporters briefly this morning.

He said that a reluctance among Russian leaders now makes it unlikely that a summit meeting with Soviet President Leonid I. Brezhnev can be held in January, as originally hoped.

"... I think we have an excellent chance of a fiarly early meeting between myself and President Brezhnev," Carter said. "My guess is, though, that it will not be in January. We would be ready in January if the Soviets are."

The president had been hoping that he and Brezhnev could sign a SALT II pact at a mid-January summit.

However, Vance and Soviet Foreign Minister Andrei Gromyko failed to tie down an agreement last week in Geneva. They siad they made substantial progress but left some issues to be resolved by SALT negotiating teams.

Asked if the Russians might be holding off until after a late January meeting in Washington between Carter and Chinese Deputy Premier Teng Hsiao-ping, Carter said:

"We don't know what the Soviets' motivations are. I talked to secretary Vance yesterday afternoon after he got back home, and he was encouraged with the meetings with the Soviets and also thought that the Isralis and the Egyptians had a good, solid discussion" in Brussels, "... It was constructive. He felt very pleased with it."

Vance went from Geneva to Brussels, where he met with the foreign ministers of Egypt and Israel in an effort to break the deadlock in the treaty talks between the two countries.

Earlier this month, the president repeatedly said that unresolved issues in both the Middle East and SALT negotiations were "minor" by comparison with the questions that had been settled.

However, Egyptian demands that the treaty be linked to a timetable for autonomy for Palestinians in the Gaza Strip and on the West Bank of the Jordan River have pushed a Middle East treaty past the Dec. 17 deadline set in the Camp David accords announced in September.

Carter declined to discuss specific issues remaining in the SALT talks. In answer to a question, however, he said there has been a partial resolution of disagreements over verfication of arms arsenals and over the Soviet practice of encoding missile telemetry to foil American analysis of test results.

The president spoke to reporters about 9:30 this morning outside the home of his mother-in-law, Allie Smith. The Carter family visited there after a predawin trip to the Pine Woods home of the president's mother, Lillian, where they ate Christmas breakfast and exchanged gifts.The Carters returned to Mrs. Smith's home for an after Christmas dinner.

The Carters elft, their home about 6:30 for the five-minute drive through fam fields and pine forests to "Miss Lillian's" modernistic home. A crescent moon and twinkling stars still shone brightly when they arrived for the celebration attended by about 20 relatives. The day itself was a classic Southern Christmas of brilliant sunshine and mild temperatures.

At Mrs. Smith's home, the president said, "Our Christmas has been very good. We hope everyone in our contry has had as good a Christmas as we have had. We have been with our families and with our friends, and we have got peace on earth right now. We hope we can keep it that way. We just wish everyone, on behalf of the First Family, a very wonderful holiday season."

Carter said his Christmas gifts included books - he carried a copy of "101 Famous Poems" - a small pocket camera, clothes and jogging suits for use "when I get recuperated from my physical injury."

The president is recovering from a severe recurrence of hemorrhoids, an affliction that continues in inhibit his usual holiday activities.

"I don't think I am goint to go quail hunting," he said. "I can't stand the thought of jumping over those terraces in a jeep right this minute."

The Carters are scheduled to remain in Plains until Tuesday morning, when they will fly to the presidential retreat at Camp David, Md. CAPTION: Picture, Carter, with wife, Rosalynn, and daughter, Amy, wishes all a merry Christmas as he leaves family breakfast.