In the waning days of the last Virginia General Assembly session, Sen. Wiley F. Mitchell Jr. (R-Alexandria) rose on the Senate floor and asked for an explanation of proposed amendments to the state employee pension law.

As he now recalls it, Mitchell restated the apparent purposes of the bill and asked if a member of the Senate Finance Committee could tell him if the legislation would accomplish any other purposes.

Mitchell said in an interview yesterday that the answer came back from Sen. Hunter B. Andrews (D-Hampton), "No, Senator. you have pretty well described the bill. That's about what it does."

The bill passed handily, but since it did, Mitchell has found that it accomplished more than he thought at the time. Tucked away in the "definitions" section of the bill was an obscure amendment inserted in the House O Delegates, that has the effect of giving legislators pension credits for travel and office allowances they receive while in the General Assembly.

By including their expense allowances in sa addition to their salaries in their pension base, the legislators added about $70 a year to their retirement income for each year of service in the General Assembly or some other state office.

This works out to about $58 a month for a five-term delegate or $70 a month for a three-term seantor.

Mitchell does not believe expense allowances should be counted for retirement income and has introduced a bill for consideration in the upcoming Assembly session that would nullify the benefit. One Sante source familiar with the bill predicted today that it would "set off a nasty, internal fight up here."

"It is ethically wrong and legally poorly advised for the Assembly to treat itself as a special class when it comes to pensions, Mitchell said. "We are supposed to be treated like other state employees in the pension program, but this bill set us up as a special category and on top of that, it applied the benefit retroactively to the beginning of 1977."

Mitchell's proposal was challenged immediately in an interview by Sen. Edward E. Willey (D-Richmond), the senior member of the Senate, its president pro tem and chairman of its Finance Committee.

"Most states have a special, lucrative retirement system for legislators," he said. "We are treated like regular state employees, but there is no way that most legislators are going to earn the maximum pension benefit even though this is damn near a fulltime job. We just don't serve long enough to get it."

Speaking of Mitchell, Willey said, "If hefeels he is overpaid for his job, then he's not doing his job. Let him sign his checks and send them back to the treasury."

Reminded that Mitchell's bill is aimed at pension credits for expense allowances, not legislators' salaries, Willey replied:

"I don't care what he says. Whatever Sen. Mitchell says, I disagree with it."

It is not the first time that Mitchell, a freshman Republican, and Willey, the senior Democrat, have crossed swords. Toward the end of the 1976 session, Willey ordered Mitchell ot leave a Senate Finance Committee meeting taking final action on the state budget. The chairman said it was opened only to members of the committee.

State law now requires that all committee meetings be open not only to all legislators but also to the public.

Whether intended or not, the pension benefit that Mitchell is seeking to nullify is of special interest to all legislators who plan to retire at the end of their current terms or are defeated in the next round of Assembly elections in 1979.

By making the benefit retroactive to the beginning of 1977, those legislators will be able to apply three full years of expense allowances to their retirement income. Pensions are based on the average income for the highest three years.

All legislators earn a salary of $5,475 a year, plus the expense allowance of $44 a day during legislative sessions and $200 a month when the Assembly is not in session. They also receive $50 a day for attending meetings between sessions.

Like other state employees, they contribute 5 per cent of their compensation to the pension fund. At 65 they receive an annual retirement income of 1.5 per cent of their average salary for the highest three years, multiplied by the years they served in office.