Nancy Reagan flew here today to pitch American support for local efforts to end the illegal narcotics trade and drug abuse, as she had done the day before in neighboring Malaysia.

Although the First Lady's visit comes at a time of political challenge to Thai Prime Minister Prem Tinsulanond -- who has called for a new election in July -- Mrs. Reagan had another focus. "I don't pay attention to that part of it, just the drugs," she said, referring to the political situation in Thailand.

Prem's role in tackling Thailand's narcotics problem is seen by U.S. officials as key to the improvements that have taken place here since 1984. After Prem and President Reagan discussed Thailand's drug problem at the White House, the Thai leader appointed a deputy prime minister to head a government committee on illegal narcotics.

Thai officials here hope that Mrs. Reagan's stop will bring in more U.S. aid to bolster the country's fight against drugs. Last year the United States gave Thailand $2.8 million for narcotics control programs, according to U.S. Embassy figures.

The first lady told reporters, however, that she has no plans to lobby for more U.S. aid and that she would not advise the president to increase assistance to combat drug trafficking in Thailand or Malaysia. But, she added: "I'll tell him about what I did."

Earlier, in the Malaysian capital of Kuala Lumpur, Mrs. Reagan visited a well protected model high school for gifted students which the government's antinarcotics task force often uses as a backdrop for discussions on drug abuse.

While its vice principal said the school is "not representative" of those attended by Malaysia's growing number of school-age drug abusers, Mrs. Reagan said she was impressed anyway. "Those kids were marvelous. The teacher was great," she said.

She also talked with parents of former drug users and said she was particularly touched by the story one mother told about her son's drug experience.

At a working lunch on drug abuse, chaired by Dr. Siti Hasmah, wife of Malaysia's prime minister, Mrs. Reagan described what happens to parent-child relationships when narcotics are involved.

"A child has a capability of hurting a parent like nobody else can ever hurt them," she said. "So no matter what country you're in, what culture you're in, this problem remains the same for parents and child."

Security precautions surrounding Mrs. Reagan's journey this weekend continued to be painstaking. As in Malaysia, where local reporters had little or no access, media coverage here is being curtailed except for journalists traveling with the first lady.

Thailand's crown prince met Mrs. Reagan at the airport. Tonight and Sunday she will be the house guest of Thailand's king and queen.

Her flight from Kuala Lumpur was delayed for about 45 minutes because of a heavy rainstorm. Once airborne, however, the first lady served reporters what she called a royal fruitcake, a gift she had received from the daughter of Malaysia's queen.

Standing in the aisle, Mrs. Reagan said she telephoned the president this morning in Tokyo and he said he was ready for the seven-nation economic summit which starts Sunday.

Asked about her telephone conversation in Hawaii last weekend with Imelda Marcos, wife of deposed Philippine president Ferdinand Marcos, Mrs. Reagan said: "She was upset."

"You know, that was supposed to be a private conversation," she said.