A 24-year-old Brooklyn mother, acccused of assaulting her 4-yearold daughter, has maintained in a novel extension of the defense of diminished capacity that her case should be dismissed because premenstrual stress prompted her to act irrationally.

According to Stephanie Benson, the Legal Aid defense lawyer on the case, the argument has not been advanced before in an American criminal court. It has been used with some success recently in mitigating sentences in Britain and has touched off debate as to its appropriateness and validity among prosecutors, doctors, judges, lawyers and feminists here and there.

Miss Benson said that last December her client, Shirley Santos, a 24-year-old single mother of six, beat her 4-year-old daughter when she refused to be quiet; the child was covered with bruises and welts. Miss Santos, distraught and remorseful, explained that she had just gotten her menstrual period. Custody Issue Raised

The misdemeanor case has been adjourned until November, during which time the Family Court, in a separate action instituted by municipal officials, could permanently remove the children from their mother's custody. That, in turn, could result in the dismissal of the misdemeanor case.

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So far Elizabeth Holtzman, the Brooklyn District Attorney, has indicated that she plans to continue the prosecution. Regardless of the outcome, Miss Holtzman predicts that the premenstsrual stress defense, known as P.M.S., will surface in other cases. Her office, she said, is gathering scientific data and expert testimony to rebut the defense.