During sexual life the non-pregnant albino rat exhibits marked rhythmic increases in spontaneous activity which are synchronous with estruation. During gestation these spurts of activity are usually absent. Occasionally, however, an estrual-like peak was found which occurred between the 12th and 15th day of gestation and indicated that estruation had taken place. This assumption is substantiated by 2 cases of superfetation which show that ovulation, fertilization, and implantation occurred about the 15th day of gestation.