The rat posterodorsal medial amygdala (MePD) is a component of the neural network that modulates male sexual behavior. Dendritic spines were counted in Golgi-impregnated bitufted and stellate neurons and from cells located in the medial and lateral MePD subregions. It was also studied the effect of 8-OH-DPAT, a 5-HT1A receptor agonist, microinjected into the MePD on male sexual behavior. There were no significant differences in the dendritic spine density obtained from multipolar bitufted and stellate neurons (n = 48 cells in each group; p > 0.05) or in the data from the medial or the lateral MePD (n = 48 neurons per region; p > 0.05). Rats were stereotaxically microinjected into the MePD with saline (0.2 microl, n = 6) or 8-OH-DPAT (0.1 and 1.0 microg/0.2 microl, n = 6 and 5, respectively). Behavioral recordings prior to surgery and "non-target" microinjections served as additional control data. 8-OH-DPAT 1.0 microg decreased the latencies to intromission and ejaculation, the postejaculatory refractory period and the mount frequency when compared to control pre-surgery data (p < 0.05). When compared among groups, 8-OH-DPAT 1.0 microg promoted the highest percentage reduction in the postejaculatory refractory period. Saline and injections in the vicinity of MePD did not promote relevant effects on ejaculation (p > 0.05). Results indicate that a similar dendritic spine density can be found in morphologically different populations of MePD neurons and, 8-OH-DPAT can facilitate male sexual behavior by acting on postsynaptic 5-HT1A receptors in this brain area.