The Montgomery County Council, questioning the cost and academic value of all-day kindergarten, is considering setting up an experimental program to provide a hybrid of half-day kindergarten and extended day care.

Council members, who are set to vote on a measure today that would establish the model programs in several elementary schools by September 1988, said they see it as a way to meet Montgomery's increasing need for day care, but at a much lower cost than an all-day academic program.

Council members stressed the experimental nature of the plan and said it was not their intent to eliminate existing all-day kindergartens. The national trend in recent years has been to establish all-day kindergarten. About 40 percent of all 5-year-olds are in full-day programs, up from 17 percent in 1970. The growth of all-day programs has been prompted by the increases in the number of working mothers and by the belief that earlier instruction improves academic achievement.

In the Washington area, however, the movement to start all-day programs has been slow to catch on. The District, Falls Church, Fauquier and Stafford counties are the only jurisdictions to offer all-day programs to all kindergartners. Montgomery provides all-day sessions to 2,376 of its 7,052 kindergartners. Prince George's County offers it in a limited number of schools.

Montgomery County school officials are strong advocates of the full-day program and would be expected to strongly resist any move to undermine it.

In a council session yesterday -- viewed by some educators as an unusual intrusion into educational matters -- members heard the opinion of a panel of educators that all-day kindergarten can be detrimental because it places too much pressure on pupils to learn and robs them of their childhood. The council had invited the panel.

School Superintendent Harry Pitt said in an interview that he favors day care and is "not against" an experimental program. But he said he was a strong believer that all-day kindergarten is an excellent academic environment.

"I know that the critics say all-day kindergarten is too academic and places too much pressure on students . . . . I argue just the opposite, by spreading out instruction over the day, instead of compressing it in 2 1/2 hours, you relieve pressure and create a better environment for learning."

The measure, sponsored by council member William E. Hanna Jr. and President Michael L. Subin and expected to be approved today, would appoint a joint school and county committee to set up a demonstration program. The program would offer a half-day of instruction from teachers and an extended child development program operated by a nonprofit day care provider under contract with the county.

Merle J. Steiner, Hanna's council assistant, stressed that the proposal is meant as alternative for parents. It would also be considerably cheaper to operate than all-day kindergarten because day care workers, many of whom are students, are paid about half the average $34,000 salary of a Montgomery County teacher.

Steiner said parents would pay for the day care on a sliding scale based upon their income. Steiner said that 50 percent of the mothers with children under the age of 6 are in the workforce, creating a tremendous need for day care that, in turn, builds into pressure for more all-day kindergarten.

Yesterday's session -- featuring Edward Zigler, director of the Bush Center in Child Development and Social Policy at Yale University; Nancy Balaban of Bank Street College in New York, and Nancy Karweit, of Johns Hopkins University -- attracted more than 100 people to the council chambers, including U.S. Sen. Barbara A. Mikulski (D-Md.) Mikulski, who has sponsored national day care legislation, said she was interested in seeing how day care is discussed and handled locally.

Subin said that the council wanted to be careful to include school officials in its discussions and didn't want to step on their educational prerogatives. Pitt and several school board members sat in on the session and, as council member Rose Crenca observed, "his body language has been screaming."

Pitt said if he had his preference and money were not an issue, he would institute all-day kindergarten across the board. Thirty-three of the county's 105 elementary schools now have some all-day kindergarten. Pitt estimated it would cost $4 million in operating costs and $8 million in capital costs to have all-day kindergarten for all 5-year-olds.