An embarrassed Senate took a dive yesterday and gave up on its plan to spend $736,400 on a new gymnasium while forcing almost everyone else to slim down financially.

Bruised by criticism over a recent refusal to deny themselves the fancy new fitness facility, agile senators reversed themselves yesterday and voted 98 to 0 to stop all spending for a planned gymnasium in the Hart Senate Office Building now being built.

In a further gesture of self-sacrifice, they also voted to stop payment on an existing small "physical fitness" facility in the Dirksen Senate Office Building. This would confine senators to the full-scale gym, complete with a swimming pool and other exercise facilities, in the Russell Office Building. These decisions were approved as a rider to a pending debt-ceiling bill that must be passed by Sept. 30 to keep the government from running out of borrowing authority. The bill is currently mired in a filibuster over an anti-abortion amendment.

Only last week the Senate voted to keep money flowing for the Hart building gym by rejecting, 50 to 48 on a point of order, a proposal from Sen. William Proxmire (D-Wis.) to prohibit spending to outfit the new facility. A public uproar followed, prompting Sen. Howard H. Baker Jr. to call on the Senate's building commission to drop plans for the gym and then, on Monday, to seek legislative action. "We're going to kill that snake one more time," he said.

The final outcome was a victory for Proxmire, but even he had to pay a price. Proxmire, a runner, is one of the few users of the Dirksen facility.