The effects of electrical stimulation to the septum on penile erections in rats were examined to clarify the mechanisms for regulation of erectile responses during different states of vigilance. Penile responses were assessed by changes in pressure in the corpus spongiosum of penis (CSP) and electromyography (EMG) of the bulbospongiosus (BS) muscle. In anesthetized and un-anesthetized rats, stimulation in and around the septum induced three erectile patterns; 1) a Normal type response, which was indistinguishable from a spontaneous erection, characterized by a slow increase in CSP pressure with sharp CSP pressure peaks associated with BS muscle bursts, 2) Mixed type response, in which high frequency CSP pressure peaks were followed by a Normal type response, and 3) a Prolonged type response, evoked only in the anesthetized rat, consisting of a single sharp CSP peak followed by a slow increase in CSP pressure and a return to baseline with multiple subsequent events repeated for up to 960 s. In addition, a Micturition type response was also observed involving high frequency CSP pressure oscillations similar to the pressure pattern seen during spontaneous micturition. We found that erections were induced after stimulation to the lateral septum (LS), but not from the medial septum (MS). In anesthetized rats, a few responses were also obtained following stimulation of the horizontal limb of diagonal band (HDB). In un-anesthetized rats, responses were also induced from the HDB and the ventral limb of diagonal band (VDB) and the adjoining areas. The effective sites for eliciting erection during rapid eye movement (REM) sleep were located in the dorsal and intermediate parts of the LS, whereas the ventral part of the LS was the most effective site for eliciting erections during wakefulness. These results suggest a functional role for penile erection in the septum, and further suggest that subdivisions of the LS may have different roles in the regulation of penile erection during wakefulness and REM sleep.