The effects of methanolic extract of Alstonia boonei
stem bark was studied on sperm characteristics, fertility, body and organ weights of adult albino rats. Oral administration of 50 and 200 mg/kg b.w/animal/day of the extract for 1, 2, 4, and 12 weeks caused duration- and- dose-dependent changes in the body weights, organ weights and sperm characteristics. However, there was a gradual increase in the weights of the liver of all treated rats which was significant (P< 0.05) in those treated for 12 weeks. Sperm viability, motility and counts were significantly reduced (P < 0.05) in rats treated for 2 and 4 weeks. Only sperm viability was reduced in rats treated for 1 week. However, rats treated for 12 weeks had normal sperm motility, viability and counts. In addition, they had normal serum testosterone concentration and fertility. Fertility was zero in rats that were treated for 2 weeks. Visible lesions in the seminiferous tubular cytoarchitecture were observed in the histological sections of the testes from the treated rats prepared at the end of the study period. Normal sperm characteristics were however, restored in rats that were allowed 4 weeks recovery period. The results therefore suggest that the extract could have reversible antifertility effects in male rats.