Silastic rods containing 25% or 50% prazosin HCl were inserted adjacent to the epididymis of rats. Silastic collars containing 25% prazosin HCl were placed around the vas deferens of rats. Treated males were infertile or subfertile for 6\p=n-\9weeks after insertion of these rods or collars. The number of spermatozoa observed in the vaginal smears was reduced. It is concluded that the antifertility effect of prazosin results mainly from inhibition of ejaculation.