Convicted hijacker Garrett Trapnell took advantage of his free behind-bars mailing privilege to solicit funds for his erstwhile presidential candiacy.

Another inmate used the unlimited supply of franked envelopes to write every member of the House and Senate - 535 in all - to complain about his problems.

There are 26,000 other inmates in the federal prison system, and while most are not running for president or flooding Congress with letters, they all have free mailing privileges.

The spiraling cost of the postage has prompted Bureau of Prisons Director Norman Carlson to propose a drastic cut in the privilege - to the equivalent of five first-class stamps a month.

The plan, to go into effect July 1, is being opposed vigorously by a group of House members.

Carlson said in a recent interview that his proposal would save the government $800,00 next year in postage - about a two-thirds cut in the current budget.

The prisons director said prisoners who could not afford to pay would be provided with free stamps, and he said that most prisoners earn some kind of salary while in the pententiary. About 6,700 inmates in one program each average about $1,100 in income a year.

Some influential members of Congress, including Rep. Robert Kastenmeier (D-Wis.), chairman of the House Judiciary subcommittee which oversees the Bureau of Prisons, agree with Carlson's approach.

But at least 19 House members, led by Rep. John Conyers (D-Mich), another House Judiciary subcommittee chairman, wrote Carlson recently to complain about the planned change in mail policy.

Some prisoners "will be effectively silenced" by the new restriction, they argued in urging the present system by saved.