Background: It was previously documented that hypogonadal men suffer from learning and memory impairment. Recent researches proved the role of the hippocampus in controlling learning and memory functions mediated by the action of gonadal hormones. However, the mechanism of this action is debatable. Aim of the Work: To verify the effects of castration and testosterone substitution on the neuronal structure of the hippocampus in adult male albino rats. Material and Methods: Twenty adult male albino rats were divided into 4 groups : control (group I), castrated nontreated rats (group II), castrated rats provided with immediate daily injections of dihydrotestosterone (group III) and castrated rats injected with daily dihydrotestosterone 3 weeks post surgery (group IV). Six weeks post surgery, all rats were sacrificed and their hippocampi were processed to be examined with the light microscope and transmission electron microscope. Results: Significant structural changes of the hippocampi obtained from Groups II and IV were noticed.  Meanwhile, rats of group III showed restoration of the normal hippocampal structure compared to that of the control. Conclusion: Castration affected the structure of the hippocampus which may explain learning and memory impairment in cases of hypogonadism. Early testosterone substitution would prevent these effects. However, late substitution showed no protective role which may indicate that hippocampal damage, when occurs, is irreversible