LOS ANGELES, FEB. 12 -- President Reagan, stopping here on the first leg of a trip that will combine business in Mexico with pleasure at his California ranch, today raised $500,000 for the state Republican Party.

Reagan attended a $10,000-a- person afternoon reception for 50 GOP donors at the home of millionaire tycoon David Murdock. The event was closed to reporters and the public, and Reagan made no speech, but he talked individually with the donors and posed for pictures with them, aides said.

Originally, Reagan had been scheduled to speak tonight at a $1,000-a-plate party fund-raiser here that has been postponed until summer.

Reagan's attendance at the Murdock event emphasized that the president remains potent as a party fund-raiser even in the final year of his presidency.

Reagan is trying to avoid public involvement in the fight for the Republican presidential nomination, but he is scheduled to make two appearances a month at GOP fund-raising events until the Republican National Convention.

The president signaled in a speech to conservatives in Washington Thursday night that he will campaign actively this fall for Republican candidates and the principles of "free enterprise and political freedom."

Saturday, Reagan is scheduled to fly to the resort city of Mazatlan on Mexico's Pacific coast for his sixth and probably final meeting with Mexican President Miguel de la Madrid, who will leave office in De-cember. The talks are expected to focus on trade and debt issues, Central America and the growing flow of illegal drugs from Mexico to the United States.

A U.S. official, who briefed reporters at the White House earlier this week, said the Mexicans had stepped up their efforts to arrest smugglers, confiscate drugs and eradicate sources of supply. Nonetheless, he said, the flow of cocaine into the United States continues to grow steadily. The official said that the Mexican government takes the view that U.S. officials are not doing enough to dampen demand for drugs in this country.

The two governments also differ on the sensitive issue of continued U.S. aid for the Nicaraguan contras, which Reagan continues to advocate. De la Madrid, who opposes aid to the contras, has raised the issue in previous meetings.

But the two presidents are expected to strike a positive note on U.S-Mexican trade relations, which have been steadily improving. U.S. Trade Representative Clayton Yeutter and Hector Hernandez, Mexico's minister of commerce and industry, will sign a textile agreement that was announced last month and perhaps another agreement alloting frequencies to mobile phones along the U.S.-Mexican border.

Reagan is due to fly directly from Mazatlan to Point Mugu Naval Air Station in California, where he will proceed by helicopter to his mountaintop ranch northwest of Santa Barbara.

He is scheduled to return to Washington Wednesday.