Female mice were mated to males that had been surgically rendered incapable of forming copulatory plugs. About half of the females were found to be pregnant, and the other half pseudopregnant, as a result of these matings. In both cases, mating with the operated males resulted in the induction of luteal activity in the female.In the normal oestrous cycle of the female house mouse, a true luteal phase is absent. Some unknown stimulus provided by the male during mating induces the luteal phase of the oestrous cycle in the female. Land & McGill (1967) initiated a search for this stimulus. They recognized three possible sources of stimulation from the male that might be either sufficient or necessary for the induction of luteal activity. These were: (a) a minimum number of preejaculatory thrusts, (b) the ejaculatory reflex and the formation of the copula¬ tory plug, and (c) the subsequent presence of the plug in the vagina. Their results indicated that up to 150 pre-ejaculatory thrusts were neither sufficient nor necessary for the induction of luteal activity. On the other hand, nine of eleven animals receiving fifty-five or fewer pre-ejaculatory thrusts, plus the ejaculatory reflex and the copulatory plug, became pregnant. A third group of animals received pre-ejaculatory thrusts ranging from twenty-four to 135, plus the ejaculatory reflex and the copulatory plug which was immediately removed. Six of these females became pregnant and eight were judged pseudopregnant.The experiment of Land and McGill failed to separate the effects of the presence of the copulatory plug in the vagina from the effects of the dramatic ejaculatory pattern exhibited by the male mouse (McGill, 1962). The present study was concerned with separating the effects of these two potential stimuli. A second purpose was to replicate and extend the findings of Land & McGill (1967).The method was essentially the same as that used by Land & McGill (1967). In brief, individual, sexually-experienced males were placed in plastic cylinders. Females were then introduced into the cylinders and any resulting matings