Twenty-four male adult albino rats, weighing between 130 and 239 g, were randomly divided into four groups of six rats per group (A to D). The rats were fed either normal pelleted commercial feed containing 14.5% crude protein alone (group A) or feed supplemented with pulverized dry leaves of Codiaeum variegatum (garden croton) at the levels of 0.25% (B), 0.5% (C), and 1.0% (D). All four groups of rats were fed for 12 weeks, during which period clean water was provided ad libitum. Every 2 weeks, body weights of individual rats were taken and blood collected for hematology (packed cell volume (PCV), total and differential leukocyte counts). Hemoglobin concentration was determined at 4-week intervals. At the end of the 12-week study period, all rats were sacrificed; testes and epididymes of individual rats were weighed separately to calculate the organosomatic index for both organs. Thereafter, testicular and epididymal sperm counts were performed. Supplementing rat feed with C. variegatum leaves in treated groups (B, C, and D) resulted in reduced weight gain such that by the 12th week of study, the mean body weight of group D rats was significantly lower than that of the unsupplemented control (group A) rats. The testicular and epididymal organosomatic index for the different treatment groups were not significantly (P > 0. 05) different from the mean values for group A rats. The hematological evaluation of the rats showed that supplementation at any of the level used did not significantly (P > 0. 05) alter the mean PCV values. The mean total leukocyte and the mean absolute neutrophil counts (by the fourth week) for the treated groups were significantly (P < 0. 05) higher than the corresponding values for the control groups. Likewise, the mean hemoglobin concentration for the treated groups was only slightly lower than the mean values for the control group for the greater part of the study period. The mean testicular sperm counts for the treated rat groups were generally lower in a dose-dependent manner, but was not significantly different (P > 0.05) from the mean values for the control group. On the other hand, the mean epididymal sperm counts for the treated rat groups were significantly (P < 0. 05) lower than the mean values for the control in a dose-dependent pattern. This study has shown that feeding male rats in particular and probably other animals in general on the leaves of garden croton at a level as low as 0.25% level of supplementation has the tendency to stimulate the immune function of animals but could severely reduce the testicular and epididymal sperm reserve counts, thereby reducing the reproductive efficiency of male animals. These results recommend further studies in respect to the immunostimulating and spermatotoxic properties of the leaves of this plant.