Virginia Gov. Charles S. Robb's proposed massive financial commitment to education, the highlight of his $18.5 billion biennial budget, came under fire today from some legislators who said Robb had slighted critical needs, including corrections and mental health.

"I'm going to vote to cut it," said Del. Franklin M. Slayton (D-Halifax), citing the $376 million increase Robb proposed as the state's share of basic education aid to local schools. Slayton, chairman of an appropriations subcommittee, is expected to press for more money for community-based mental health programs.

Some of the legislature's more liberal members also questioned the size of Robb's education package. "I was concerned all along," said Del. Warren G. Stambaugh (D-Arlington), "that this was exactly what was going to happen; that because of the big push for education every other essential service was going to suffer. Education is important, but education is not the only important thing in Virginia."

The fight over education funds, however, is one that Robb won't have to wage. He leaves office at noon Saturday and his successor, Democrat Gerald L. Baliles, will have to defend the two-year budget before the 1986 General Assembly. The amount of state aid to schools is expected to be second in controversy only to how to finance highway construction during the 60-day session.

Baliles, who campaigned on a promise to continue Robb's education programs, is expected to call for greater expenditures for welfare recipients. They would get no increases under the Robb budget. Baliles will send his suggested revisions to the budget to the legislature Monday.

Incoming governors in Virginia typically have relatively little impact on their first budgets. Robb's budget, for the biennium beginning July 1, spends all but $1.8 million of the estimated $18.5 billion in revenue that the state expects to receive in the budget period.

Details of Robb's final budget were first disclosed this afternoon, and its impact on the state's localities could not be immediately determined. Officials could not say today what impact the budget's new education aid formulas would have on the state's richest school systems, such as Fairfax and Arlington counties, which pay higher salaries and employ more than the 59.5 teachers per 1,000 students that the state regulations require.

While some Northern Virginia officials predicted earlier that their schools could lose money under the new changes in education funding, legislators now say they believe those fears may have been unfounded. Legislators and local government lobbyists, who first saw the budget document today, said they had not yet been able to determine the bottom line on the payments to local districts.

State Sen. Wiley F. Mitchell (R-Alexandria) said some districts in Northern Virginia could receive more aid and others less than they now receive because of the complexity of education aid formulas, which are based on real estate property values, student class sizes and the size of local financial contributions to school systems.

State agencies themselves are divided over how much it will cost for the state to fully fund the cost of meeting its mandated educational standards -- the Standards of Quality. Baliles made state funding of the standards one of his top issues in the fall campaign.

The state Department of Education has said the standards could be met with $395 million more in state funds but the Joint Legislative Audit and Review Commission, using a different set of figures, put the cost at about $192 million.

Robb said he agrees that the formula needs to be changed, but it would be asking too much for local school systems, which have been expecting much larger amounts of state aid this year, to immediately revise their budgets to the lower level of state aid. So Robb said he added $184 million to the commission's figure, saying the extra money could be used to help school systems adjust to new levels of state aid.

Robb's plan also includes controversial provisions to force poorer school systems to use increased state money for salary increases rather than to divert it to other operational costs, as many have done in the past.

Under the plan, the school systems would have to demonstrate that they applied the first year increases to teacher pay raises before being eligible for increased state aid in the second year of the two-year budget cycle.

School systems, including those in Northern Virginia, that pay teachers an average of more than $24,547 would be exempt from the requirement.

Projected revenue for the state's general fund, which comes mainly from five taxes -- individual income, corporate income, general sales and use, gross premiums on insurance and public service corporations -- was placed by Robb at $9.3 billion, an increase of 15.9 percent over the current two-year level.

The other $9 billion is money that comes primarily from grants and donations, and tuition and service charges.

The budget allocates $188.5 million to state employes in an effort to maintain pay parity with private industry.

Each of the state's 88,000-plus classified employes will get a 4.57 percent pay raise; two-thirds of them will share in $66 million set aside for merit increases that average 4.5 percent, and 20 percent will get $1,000 bonuses for "exceptional performance."

Agency heads will get a 6.5 percent pay boost, but the biggest pay raise of all will go to the legislators themselves, with Robb's proposing that their annual salary be raised from $11,000 to $18,000 in 1988.

The biggest single item in the $28.4 million corrections budget is $10.4 million for the new Augusta Correctional Center, scheduled to open next month.

The budget projects that the prison system will add 500 inmates, which will require hiring 110 employes while expanding the parole program at a cost of $6.1 million.

The proposed Robb budget includes major new appropriations for Northern Virginia, including funding for 29 additional state troopers to patrol suburban Washington highways and $350,000 to promote the use of Dulles International Airport.

The region's colleges and high technology industry also would receive a boost with the state financing 55 new positions at George Mason University and contributing $1.8 million toward rental expenses of the developing Software Productivity Consortium, a group of high tech industries in the Dulles area.

Robb did propose $3 million for a zoo in the Roanoke area, contingent upon matching funds from businesses and governments there, a project Baliles supported in his campaign.

A $1.5 million allocation to expand the home-delivered meals program reflects the fact that the elderly are the fastest growing age group.

By the year 2000, persons over 65 will account for 11.5 percent of the total population, up from the present 9.5 percent.

One age group that will decrease is what the budget calls the "crime-prone" group, ages 18 to 34. Nonetheless, the Corrections Department, which has been the bane of the Robb administration, will get $28.3 million in the new budget.

Capital projects will cost $174.3 million, including $4.2 million for a forensic laboratory in Northern Virginia; $2.1 million to replace a motor vehicle branch in Fairfax; $700,000 to expand the one at Tysons Corner, and money for a new state guard armory in Alexandria and renovation of the existing one in Manassas.

The massive multistate effort to clean up the Chesapeake Bay, whose pollution threatens the important seafood industry, will benefit from a $3.3 million increase in the Virginia budget, for a total of $20.6 million.