Rhodesia announced today that it will call up all ablebodied men between 38 and 50 for military service and will tighten up draft exemptions.

The decision was met by warnings that it could lead to an economic collapse that could bring down the government sooner than the continuing civil war.

[The Organization of African Unity said Thursday in Lusaka, capital of Zambia, that black African states would not object if Rhodesian blacks call on "Cuba or other foreign power" to help topple the white regime, United Press International reported. Cuban troops aided the victorious Popular Movement for the Liberation of Angola in that country's civil war. Monday Rhodesian Prime Minister Ian Smith rejected a British settlement plan.]

The Rhodesian callup is designed to strengthen the government's campaign against growing guerrilla forces. Defense Minister Reg Cowper said tonight that deferments and exemptions for men under 38 will be canceled "almost immediately." He said he will announce plans for the callup - most likely on active reserve duty - of men between 38 and 50 within the next few weeks.

The decisions affects all-white, Asian and "colored" (mixed-race) males. Blacks, who make up 66 per cent of the army and 75 per cent of the police, are all volunteers.

There is growing concern in the capital that the callup, which will affect many prominent figures in commerce and industry, may be the final blow to the already troubled economy. There is also concern that the callup will lead to further white emigration, again draining manpower. Some businessmen said privately that the resulting disruption could collapse the government.

Business leaders made two last ditch appearls to Cowper this week to review the decision because of the inevitable drain on skilled manpower.

The economy has been gravely affected by the callup of all men between 18 and 38, resulting in a manpower loss that has cost local businesses several million dollars, local merchants, bankers and manufacturers claim.

One plant that makes agricultural machinery has lost more than $400,000 in the past year bacause 120 of its technicians and managers - 60 per cent of its skilled staff - have been on reserve duty or police callup.

A branch of Barclays Bank in Salisbury has lost 18 of 20 white officials to current callups. Many firms are already attempting to run at less than half strenght.

The situation is aggravated by the government requirement that businesses make up the difference between a man's low service pay and his normal salary or wages.

The callup of the last group of eligible men is designed to strengthen the army and police campaign to contain the four-year-old guerrilla war. Diminishing the threat of the rapidly escalating confrontation will, the government hopes, allow time to establish majority rule through negotiation with moderate Africans and build up Rhodesia's international barganing power.

The level of the war was reflected in a military communique issued simulaneously with Cowper's statement. In the past two days, one white farmer, two police reservists and 25 guerrillas have been killed, the government said.

Every white loss has a serious impact on morale of whites here, out-numbered more than 20 to 1 Africans. The Smith government's new settlement initiative depends on bolstering white morale.

In the past few months, guerrillas have penetrated deep into all five "operational" or war zones, escalating the war from border skirmishes to major assaults.

Rhodesian security forces have made their gains through punitive raids across the border into Mozambique, where the majority of guerrillas are trained and based.

But with fronts from neighboring Zambia and Botswana now open, all but 138 miles of Rhodesia's 1,482-mile border are now vulnerable, stretching security forces thin.

In a radio interview last week, Cabinet official Ted Sutton-Pryce acknowledged that about 2,000 guerrillas are now operating inside Rhodesia, with perhaps 9,000 more training in Mozambique, Zambia, Botwana and Tanzania.

On nationwide television, Cowper said"We're playing for high stakes . . . Government has decided that the stage has been reached where the needs of the security forces must be paramount."

He said the turning point was the decision by five African presidents to back the "Communist" Patriotic Front, the allience between two militantly socialist Rhodesian nationalists, Robert Mugabe and Joshua Nkomo.

The defense chief said that Rhodesia has the manpower to stand up to the Patriotic Front guerrillas if the "thousands of men" who had been granted exemptions were recalled and the older categories called up.

Prime Minister Ian Smith hinted at the new callup during his television address Monday when he announced rejection of the British settlement plan and the decision to open new negotiations with "moderate" blacks inside the country.

"There are additional plans in the pipeline," he said, "and in general terms, a new and tougher line is being considered for our security effort. We are making preparations to insure that we are in a position to counter any increase in terrorism."

The prime minister received one important vote of confidence today for his decision to go it alone with internal negotiations on majority rule and elimination of racial discrimination.

Party chairman Des Frost indicated earlier this week that he and other party executives had strong objections to the elimination of all racial discrimination laws but today they agreed to support Smith's decision.