The $90 million that House and Senate incumbents spent this election year on franked mass mailings to constituents could grow in the year ending next Sept. 30 to $94 million, the amount included in the fiscal 1991 legislative branch appropriations bill.

But chances are it won't.

For the first time, House members must report publicly how much mailing they do, and the spotlight is expected to shrink what until now has been House members' growing use of taxpayer-financed mailings. Members of Congress also will be operating under tighter rules, approved last month after criticism that the frank -- the privilege of sending mail free under a lawmaker's signature -- increasingly has been used for campaign purposes.

The effects of public disclosure can be seen in the case of Sen. Larry Pressler (R-S.D.).

In the first three months of last year, when Senate mass-mailing limits were temporarily lifted, Pressler sent out so much franked mail in his state that his per-constituent total was higher than any other senator's.

His reports showed he mailed almost 3 million pieces, an average of four pieces for every man, woman and child in the state, at a cost of almost 75 cents in postage for every constituent -- about $400,000 in all. The mailings included five statewide newsletters, dozens of "town meeting" notices, and specialized first-class letters to small businessmen (22,000), pork farmers (5,972), veterans (10,882) and a handful of other groups.

But after news coverage of Pressler's mailings, based on the two pages of listings that appear every six months in the two-volume, 1,830-page Secretary of the Senate report, Pressler all but quit using mass mailings. In 1990, he mailed just one statewide newsletter.

Under a new Senate rule, members now will be required every three months to file for public review copies of each piece mass-mailed to 500 or more constituents, and the costs will be published in the Congressional Record. In the past, those filings were made two months after the close of the calendar year.

House members, who in the past repeatedly refused to go along with the Senate on public reporting of mail spending, now not only will have the costs of their mailings published every quarter, but for the first time they will have to file for public review copies of all mass-mailed materials. In the past they only had to disclose those pieces sent via postal patron delivery.

Other rules and practices will work to cut down incumbent mailings.

Although the Senate increased its franking appropriation from $24 million last year to $30 million this year, the members put in place two new rules that may make it less likely those funds will be used.

On the average, members need less than 20 percent of their mail allocation to answer letters received from constituents. About one-third of the senators send few if any mass mailings, according to past filings. Another third of members on occasion have taken advantage of a Senate rule that allowed them to "borrow" mail funds from colleagues.

That practice was voted out last month. In addition, senators who don't use their entire allocation in the first year of a congressional session will be able to carry the balance over only into the second year of a session, and no further.

The House's new rules prohibit a member from sharing his or her allocation or carrying it over from one year to the next.

While Congress may have taken steps to limit its mailings, two other costly, taxpayer-financed incumbent tools -- satellite-transmitted television spots and computer-driven electronic mailings -- will not be subject to public disclosure.

The first steps toward tightening the rules on such operations have been taken, however. A House amendment, passed on the last day of the session, would require the Senate to halt its practice of subsidizing official expense accounts with campaign funds, a practice that pays for a good part of each senator's satellite transmission of television spots.

An amendment to the legislative appropriations bill by Sen. Ted Stevens (R-Alaska) would have mandated public disclosure of computerized, electronic press releases. However, in a last-minute move, Senate managers of the measure did not ask that Stevens's amendment be included in the final House-Senate conference report.