While only 25 months ago he vowed that black rule would "never in a thousand years" come to Rhodesia, Prime Minister Ian Smith is now hard at work trying to persuade the white minority to accept the inevitable.

Smith is attempting to explain how and why he bowed to black majority rule. And he is telling increasingly apprehensive whites that the constitutional guarantees he won for them in three months of negotiations with black leaders are the best they can expect to get.

Statements like this are being made to closed meetings of whites by Smith and his minister of foreign affairs and information, P. K. Van der Byl, in an attempt to "sell" the bargain struck with three moderate black leaders Mar. 3 - the so-called Salisbury agreement.

The white leaders want to reassure the whites that the agreement, which calls for black majority rule by the end of the year, will not mean a major upheaval in their lives despite the fact that it will end 90 years of white minority government.

Although the meetings were closed to the press, accounts of the sessions have been given to reporters by people who attended.

Van der Byl reportedly told white audiences that the government had failed to win the guerrilla war (although he said this does not mean it is defeated) and that the Salisbury agreement was a "masterpiece as a political-diplomatic exercise."

Other remarks made by the two white leaders included ressurances that the standard of living of whites would not change all that much, that Dec. 31 was probably too soon for independence since elections could not practically be held before then, and that the new name for Rhodesia will not necessarily be "Zimbabwe" - as all blacks now presume it will be.

Some whites described the sessions as "extraordinarily candid and forthright." But other whites and some blacks say the Smith government is "not being honest." because it has failed to make whites realize that major changes looming in Rhodesia's future require alterations in their traditional attitudes.

For example, when asked when he would eliminate legislation which segregates residential areas, Smith said blacks were happy living where they were. He added that there would be many problems for the transport system if such changes were made, according to two participants at the meeting. One of the objectives of the Salisbury agreement is to eradicate all racial discrimination.

Van der Byl and Smith laid responsibility for the advent of majority rule on Rhodesia's guerrilla war, on a worsening economic situation and on pressures from "our friends," especially South Africa.

"We have failed to win the war which has spread to the whole of the country. We have not been defeated. We could have gone on alone without defeat but for the turn of events in Mozambique and Angola," according to notes taken by one listener at Van der Byl's speech, and later confirmed by him.

Both Smith and his foreign minister said former secretary of state Henry Kissinger's 1976 prediction that each delay in accepting majority rule would mean increased pressures and a worsening of their position had proven correct.

"However much we dislike it, we must face the reality. Even South Africa, the last citadel against the black tide, has conceded majority rule in South-West Africa(Namibia)," Van der Byl reportedly said.

But, he pointed out, "our advantage is that whereas we (whites) were alone, we now have the advantage of authentic black nationalists defending our political position. And they are not just tame blacks."

Van der Byl later said his point was that the "whites were now participating with black nationalists who have the overwhelming support of the majority."

"No one ever believed that we could get internal (black) leaders to agree to so much," Van der Byl said, according to one white. "True, we wanted 34 seats. But 28 is more than the rest of the world would have conceded. The Anglo-American proposals gave us only 10," he reportedly said, referring to the U.S. British plans for a Rhodesian settlement that would include the guerrilla leaders.

Van der Byl confirmed in an interview that he did say "something like this," but he added that "no one had ever believed that the Rhodesian whites would go as far as we had."

Van der Byl and Smith explained to the closed meetings how the so-called "blocking mechanism" will give whites a veto over any legislation that is inimical to their interests. This "bloching mechanism" is the crux of the Salisbury accord and the patriotic Front leaders say it negates true majority rule.

Under the agreement, at least six white members of parliament must vote with 72 black members to alter the civil service board and its pensions, the army, judiciary, police and a yet-to-be written bill of rights. The assumption is that army, police and judiciary will remain white-dominated and therefore will protect white interests.

When the prickly question arose from the audience on how whites could be sure the new constitution would not be torn up by a new black government, Smith reportedly retorted angrily, "I cannot give you iron-clad guarantees. I wont be in the next government because all I want is to retire to my farm."

Van der Byl is reported to have told whites that although their standard of living would drop, life would still be acceptable. He gave the example of the Ivory Coast as a place "where whites do make a contrubution and live in amity with a black majority."

Some participants said Smith and Van der Byl encountered hostile questions at times. One person charged that the whites were virtual "prisoners" in Rhodesia because they could only take out the equivalent of $1,150 if they wanted to emigrate. Another asked why the war was getting lifted if there had been a concession to majority rule.

Smith is said to have lost his temper more than once and to the last question he replied that he had never promised these things would happen "overnight".