Ocimum gratissimum Linn.(OG) is widely used in folk medicine for several conditions because of its high medicinal value and therefore calls for its toxicological screening. The present study is designed to investigate the effects of aqueous leaf extract of OG on hormonal and semen parameters of mice. Animals were divided into 3 groups and orally administered OG (11-88 mg/kg) daily for 1, 2 and 4 weeks, respectively, while a fourth (control) group received only distilled. They were sacrificed at the end of each treatment period and blood sample was assayed for serum levels of testosterone, FSH and LH. Semen was collected and analyzed for semen parameters, while the testis was evaluated for histological changes. OG caused no significant (p≤ 0.05) effect on the serum levels of the hormones studied. However, sperm count and motility were decreased, while the percentages of abnormal sperm cells, sperm debris and primordial cells were increased dose-and time-dependently. Furthermore, OG caused histopathological damages to the seminiferous epithelium of the testes. The results indicate that OG has anti-fertility effect in the male mouse, which may be mediated through a direct deleterious action on the testis without disruption of the testicular endocrine function.