Abstract: The present study is intended to investigate the dose-and time-dependent effects of the aqueous crude extract of O. grtissimum Linn. leaf (OG) on some biochemical and hematological parameters in the male mice. Animals were grouped into 3 (each containing 4 groups, n = 5) and orally administered with OG (11-88 mg kg-1) and observed for 1, 2 and 4 weeks, respectively. A fourth group was used as the control and given only distilled water for 4 weeks. While the serum levels of alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, creatinine and total cholesterol were not significantly (p≤0.05) different in experimental animals compared to control animals, 1 week administration of OG significantly decreased the serum levels of urea and total protein in a dose-dependent manner. Furthermore, uric acid, total acid and prostatic acid phosphatases were increased during the fourth week of administration in OG-treated animals, compared to control animals. In addition, the first and second weeks of OG administrations resulted in dose-dependent reductions in PCV, Hb and neutrophils, with increase in WBC and lymphocyte counts. However, administration of OG for 4 weeks had no significant effects on the hematological parameters. The study shows that OG will cause anemia and may affect prostatic/testicular function in the mouse, depending on the duration of exposure to the animal.