The Reagan administration has concluded that Cuban President Fidel Castro is "not serious" in proposing to stop all forms of military aid in Central America if the United States will do the same, according to well-informed official sources.

"There's no plan to negotiate with Mr. Castro," said one high official who requested anonymity. "Until the Cubans stop what they're doing in Nicaragua, there's nothing to negotiate."

The official said that Cuba had continued to supply heavy weapons and other aid to the Nicaraguan military forces ever since Castro proposed withdrawal of all foreign forces from the region two weeks ago.

According to the official, information received by the Reagan administration shows that Castro is "highly nervous" about U.S. military exercises in the region but has not given "any indication" that he is genuinely interested in negotiations that could lead to a political settlement of armed conflicts in El Salvador and Nicaragua.

In recent weeks Reagan has muted his denunciations of the Soviet-Cuban "war machine" and stressed the importance of economic assistance to Central America and diplomatic negotiations. He emphasized these themes again in his weekly radio broadcast today.

But administration officials said that the president is expected to express skepticism about both Cuban and Nicaraguan intentions in conversations scheduled Sunday with Mexican President Miguel de la Madrid in La Paz, Mexico.

Mexico is a force in the Contadora group, which also includes Panama, Colombia and Venezuela. Last month the Contadora group proposed a 10-point peace plan for Central America. Soon afterward, Daniel Ortega, the coordinator of the ruling Sandinista junta in Nicaragua, offered a similar plan.

"The president will express his support for the Contadora principles, but he wants the Contadora nations to act in good faith," one official said. "We don't want them to play into the hands of the Sandinistas and give them the respectability they badly need in the eyes of the world."

Reagan's radio address today, which was on Latin policy, made no mention of any of the peace proposals, but he said his administration was "working hard to provide economic and political support for development so that ballots will replace bullets in that troubled region."

His speech, delivered from a hotel room here, acknowledged that both the mail received by the White House and public opinion surveys show opposition to administration policies in Central America.

Reagan read a composite letter his staff put together that he said "combines the most widespread misconceptions." It said:

"Dear Mr. President:

"The U.S. has not learned any lessons from history. We refuse to understand the root causes of violence and revolution. El Salvador proves that we continue to support ruthless dictators who oppose change and abuse freedom. And by refusing to deal decently with the Sandinista government in Nicaragua, we have forced it into the arms of Cuba and the Soviet Union.

"Military measures will just make things worse. Anyway, democracy isn't, can't work in Central America."

Reagan responded by saying that "we have learned from history," and asserting that is why U.S. economic assistance to the region is three times greater than military aid.

The president said that the United States is not supporting dictators but cooperating with the "true democracies" of Costa Rica and Honduras and working to help El Salvador become one.

Then Reagan repeated his frequent assertion that the Sandinistas seized power in Nicaragua through a revolution and then "betrayed their repeated promises of democracy and free elections."

He made no mention in his composite letter of the opposition expressed by many Americans to CIA financing of the anti-Sandinista guerrillas in Nicaragua, but he indicated his support for the rebels in another passage of his speech.

"We support the elected government of El Salvador against Communist-backed guerrillas who would take over the country by force, and we oppose the unelected government of Nicaragua, which supports those guerrillas with weapons and ammunition," Reagan said. "That, of course, puts us in sympathy with those Nicaraguans who are trying to restore the Democratic promises made during the revolution, the so-called Contras."

Later, in a speech to the American G.I. Forum, Reagan said that the people of Central America will "never have a chance to build their future of freedom and prosperity if they are surrendered to the tender mercies of Fidel Castro, Libyan leader Col. Muammar Qaddafi, and their superintendents in Moscow."

"We can't let that happen; we won't let it happen," Reagan said.