The protein product of the immediate-early gene, c-fos, was visualized immunocytochemically in forebrain neurons of gonadectomized male and female rats which were injected daily with testosterone propionate (TP) and either tested for mounting directed toward a sexually receptive female or left alone in a test arena. Lidocaine anesthetic paste was applied to the genital region of all subjects in an attempt to reduce the incidence of intromissive behavior patterns. In this way we hoped to compare the relative contribution in the two sexes of vomeronasal/olfactory, as opposed to genital/somatosensory, stimuli to mounting-induced forebrain Fos immunoreactivity (FOS-IR). Males displayed high levels of mounting, with very few intromissions; females displayed a similar level of mounting coupled with a significantly higher number of intromissive behavior patterns than males. Significant increments in the number of FOS-IR neurons were seen in the medial amygdala (mAMYG) and medial preoptic area (mPOA) of males and females killed 1 h after testing. In experiment 2, ovariectomized, TP-treated females were given a unilateral lesion of the olfactory peduncle and subsequently tested with an estrous female as in Experiment 1. Unilateral lesions significantly reduced the number of FOS-IR neurons counted in the ipsilateral piriform (primary olfactory) cortex, but failed to attenuate the ability of stimuli associated with mounting an estrous female to augment FOS-IR in ipsilateral mAMYG or mPOA neurons. The results suggest that vomeronasal/olfactory stimuli were primary determinants of the mounting-induced increments in neuronal FOS-IR observed in males, whereas in females a combination of genital/somatosensory and olfactory/vomeronasal stimuli account for the observed induction of forebrain FOS-IR.