Recent studies suggest that in female monkeys and rats, estrogens elicit dendritic spine synapse formation in the prefrontal cortex, an area that similar to the hippocampus, plays a critical role in cognition. However, whether gonadal hormones induce synaptic remodeling in the male prefrontal cortex remains unknown. Here we report that gonadectomy reduced, while administration of 5-dihydrotestosterone or estradiol-benzoate to castrated male rats increased the number of medial prefrontal cortical (mPFC) spine synapses, with estradiol-benzoate being less effective than 5-dihydrotestosterone. To investigate whether the androgen receptor contributes to the mediation of these changes, we compared the response of testicular feminization mutant (Tfm) male rats to that of wild-type animals. The number of mPFC spine synapses in gonadally intact Tfm rats and 5-dihydrotestosterone-treated castrated Tfm males was considerably reduced compared to intact wild-type animals, while the synaptogenic effect of estradiol-benzoate was surprisingly enhanced in Tfm rats. These data are consistent with the hypothesis that remodeling of spine synapses in the prefrontal cortex may contribute to the cognitive effect of gonadal steroids. Our findings in Tfm animals indicate that androgen receptors may mediate a large part of the synaptogenic action of androgens in the mPFC of adult males. However, because this effect of 5-dihydrotestosterone is not completely lost in Tfm rats, additional mechanisms may also be involved.