Summary. In 4 experiments, various striated penile muscles of the rat were excised. Without the ischiocavernosus (IC) muscles no dorsiflexions ('flips') of the glans penis occurred during ex copula reflex tests, but erections were unaffected. In attempted copulation, males lacking the IC muscles rarely gained intromission, apparently because dorsiflexion of the glans penis is necessary for penetration of the vagina. Nonetheless some males lacking the IC muscles displayed the gross motor pattern of intromission and ejaculated, but rarely within the vagina. Males lacking the bulbocavernosus (BC) and levator ani (LA) muscles were incapable of developing intense erections ('cups') in ex copula tests, but they did have lesser erections, probably due to vascular action. Males with excised BC and LA muscles displayed normal copulatory behaviour, including intromission and intravaginal ejaculation, but only 1/15 females mated to these males became pregnant. The infertility of the males was attributed in part to their inability to form the penile cup, which caused them to withdraw a larger portion of the seminal plug from the vagina and, presumably, prevented the plug from being tightly lodged against the cervix. In male rats copulation apparently requires co-ordination of the penile vasculature with the contraction of separate groups of striated penile muscles, each having a distinct contribution to the integrated pattern of copulation and, ultimately, to the male's fertility.