The effect of (D,L)-propranolol at various concentrations (3.4 X 10(-6) to 3.4 X 10(-3) M) on human spermatozoal motility and zona-free hamster ova penetration was studied in vitro. Propranolol, in 10(-4) M concentration, completely abolished the spermatozoal motility and hamster ova penetration. No significant effect was observed on the spermatozoal motility and the hamster ova penetration efficiency with concentrations of the agent as low as 3.4 X 10(-6) M. The blocking action of propranolol on the in vitro penetration of hamster ova appears to be a direct consequence of the depressive effect of this drug on spermatozoal motility.