Neutralization of pituitary prolactin by the administration of antiserum to prolactin in the mouse selectively blocked the pro-oestrous stage of the oestrous cycle and inhibited ovulation in normal cyclic mice. In pseudopregnant mice, the antiserum shortened the duration of pseudopregnancy. Failure of the antiserum to affect uterine cell division, blastocyst implantation and pregnancy suggests that the secretion of progesterone by the luteal tissue appears to be more influenced by LH than by prolactin.