The hard-fought labor contract between United Parcel Service and the Teamsters union hit aconstitutional snag in the vote tally from the rank-and-file, forcing negotiators back to the bargaining table.

On Monday, Teamsters union leaders - previously confident that their four- year contract would pass the 165,000-member UPS rank-and-file - acknowledged that the pact hit a brick wall.While the national contract was approved by more than a 2-to-1 margin, six of the 16 regional supplemental agreements were rejected. As a result of a constitutional Teamsters amendment passed in 1991, all supplements must be approved for the master UPS contract to be ratified.

The UPS national contract was approved by a vote of 61,387 to 30,640 in ballots received in the mail for the last month.

Ironically, it was then-outsider Ron Carey, who pushed the constitutional amendment to give local units more power against the Teamster hierarchy. Today, Mr. Carey, as president of the Teamsters international, had to ask both sides back to the bargaining table.

"There is no national contract until we can get these things worked out," said Mario F. Perrucci, chief negotiator on the Teamsters UPS national negotiating committee. "We're going to go back and get the parties together and see if we can work out these issues that seem to be the sticking points."

While a contract extension to which both sides agreed remains in effect, Mr. Perrucci said the union was not willing to be patient. The Teamsters, he said "will take a hard look at job actions if these things cannot be worked out."

"But we are going to see what those issues are before we would keep 165,000 members from working. They may not be critical," he said.

Before 1991, supplements were negotiated independently by regions, with national contract stipulations generally superseding such pacts.

Regional supplements deal with everything from work rules, uniform reimbursements, hiring scale for seasonal help to pension contributions. Each varies depending on local conditions, and union leaders said they would evaluate each to address members' complaints.

Ken Sternad, a spokesman for UPS, acknowledged that some regional supplements had not passed. But he said "no one is talking about job actions, and we feel it is business as usual. It's a done deal."

Mr. Perrucci said he expected UPS to take court action to press its claim that the contract is approved, if it cannot reach an agreement on the supplemental pacts.

"I would say that the company would take a legal stand that since the national contract was ratified, it would not have to be bound by the regionals," he said.

A spokesman for the Teamsters said the union would likely sit down this week with UPS and renegotiate the regional supplements, then vote in those regions. No new national referendum would be conducted, said Bernie Mulligan, the spokesman.

The supplemental agreements were rejected by a wide margin in western Pennsylvania, and in closer votes central Pennsylvania, a portion of Maryland, Puerto Rico, New Jersey, and the Atlantic area, which stretches from Baltimore to South Carolina, Mr. Perrucci said.

He said he asked UPS not to pay the retroactive pay and benefits due to members since the expiration of the last contract on July 1. But the chief negotiator said he expected the company to claim that the contract was complete.

The Teamsters for a Democratic Union, a key union faction that earlier supported Carey in his efforts to win the union presidency, said it supported the union's position.