We addressed the question of how rapidly gonadal steroids might affect behavior by studying how fast testosterone (T) could augment the actions of the striated penile muscles and their associated penile reflexes. Eight male rats, functionally castrated 4 months before this study, bore chronically implanted electrodes in the bulbospongiosus (bulbocavernosus) muscle. The males were observed for the display of penile reflexes immediately after the injection of T (250 µg i.m.) and T propionate (250 µg s.c), as well as after injections of only the oil vehicle. Overt penile responses were rare. However, in several tests subcutaneous twitching was observable near the midline posterior to the penis. These twitches were accompanied by electromyographic bursts and were attributed to contractions of the bulbospongiosus muscle. T reliably (p < 0.025) accelerated the onset of electromyographic activity: 6 of the 8 males had electromyographic bursts before the 30-min limit, and 3 males responded within 6 min. This is the first demonstration of such a rapid action of androgens on behavior or its basis in striated muscle activity. The rapid muscular response to T was ascribed to steroid-sensitive neuronal membrane receptors. Such responsiveness could increase the intensity of penile reflexes within minutes after surges of luteinizing hormone and T have been induced by cues associated with estrous females and thereby could contribute to the fertility of matin