Male mice were forcibly removed from oestrous females at times ranging from 1 to 7 sec following the beginning of the ejaculatory reflex. Females were then tested to determine if luteal activity had been induced. The results indicate that the stimulus inducing luteal activity in the female is initiated 3 or 4 sec after the beginning of the ejaculatory reflex. Observations of the penis during this reflex revealed a large increase in the diameter of the erect organ, particularly at the distal end. This anatomical change continued for 5 to 12 sec. It was suggested that the luteal-inducing stimulus is the mechanical stretching of the vagina and/or the cervix which results first from the swelling of the penis and is normally continued by the copulatory plug. Speculation concerning the events occurring during the transport of spermatozoa from the male to the uterus of the female is reported. Possible methods of improving the current techniques of artificial insemination in mice are discussed.