RICHMOND, JULY 31 -- After a month of negotiation and numerous deadline extensions, A.H. Robins Co. agreed today to Rorer Group Inc.'s proposal to merge the pharmaceutical companies.

Roscoe Puckett Jr., a spokesman for Robins, announced the decision in a five-paragraph statement just hours before a midnight deadline on the offer.

The merger will become part of Robins' plan of reorganization under Chapter 11 protection from creditors that has been submitted to U.S. Bankruptcy Court in Richmond.

"We are pleased to report that negotiations relative to a definitive merger agreement between our companies have been fruitful," E. Claiborne Robins Jr., Robins' president, and Robert E. Cawthorn, Rorer's chief executive officer, said in a statement. "The board of A.H. Robins will meet as soon as possible next week to consider the agreement."

IF the board approves the agreement, the 122-page agreement will be submitted to the bankruptcy court, according to the executives.

Puckett declined to give details of that agreement until the court has it in hand.

The companies, which still were completing a number of schedules and other supporting documents tonight, said Robins must file a second amended reorganization plan and disclosure statement by Aug. 21 to incorporate the merger agreement.

In August 1985, the Richmond-based Robins filed for protection against mounting claims from women who alleged damage to their health from use of the Dalkon Shield, an intrauterine birth control device marketed by the company worldwide during the 1970s.

Earlier, Robins had proposed a trust that would pay a maximum of $1.75 billion for the claims in a plan that would keep the company independent.

The Pennsylvania-based Rorer also had proposed to establish a $1.75 billion trust to settle more than 300,000 lawsuits stemming from Robins' Dalkon Shield, and to pay full value for Robins' business creditors.

{Washington Post staff writer Morton Mintz reported that Robins has asserted that the merger is in the best interest of the bankrupt company and therefore of Dalkon Shield victims.

{But in recent court papers, the Dalkon Shield claimants' committee called the assertion "without merit."

{"The merger is subject to a number of serious contingencies," Murray Drabkin, the committee's lawyer, wrote. "It is unlikely in the extreme that it (the plan) will ever become effective."

{Drabkin said the chief contingency is confirmation of a Rorer-Robbins financial reorganization plan based on the merger, Drabkin said. "Such a plan manifestly could not be confirmed," he said, partly because it would violate the bankruptcy code by paying the owners of Robins before paying Robins' debts.}