The contraceptive properties of two adrenergically active drugs, propranolol and reserpine, were measured in vivo and in vitro. Propranolol became effective as a spermicide for human sperm at a concentration of about 2 x 10(-3) M and for rat sperm at 2 x 10(-2) M. Reserpine was also more effective on human than rat sperm and about 20 times less effective than propranolol. Both propranolol and reserpine prevented implantation when instilled into the uteri of rats. The anti-implantation effect persisted for up to 5 days after instillation of propranolol. When freely breeding rats eventually became pregnant after daily vaginal instillation of propranolol, the number of uterine implantations decreased as the concentration of vaginally instilled propranolol increased. The implantations tended to be toward the fundal end of the uteri, away from the source of propranolol, thus indicating a topical rather than a systemic anti-implantation effect.