Garlic (Allium sativum) is a popular spice usually eaten raw or cooked in various doses and its potential medical properties have been recognized for thousands of years (Agarwal, 1996). Not much has been reported on the side-effects, particularly on male reproduction, of a garlic treatment. The few reports in the literature are conflicting. Testosterone is a steroid hormone from the androgen group. 21 adult male wistar albino rats (250-350g) were randomly divided into control(n=7), test 1(n=7) and test 2(n=7) groups. The test 1 and test 2 groups received oral doses of 1.4 and 2.4 ml respectively of 100 mg/ml garlic extract for 31 days. The aqueous garlic extraction was by the maceration process and feeding was through an orogastric tube. Their body weights were measured before and after the study. They were sacrificed and blood samples collected by cardiac puncture, while serum testosterone was assayed by radioimmunoassay technique using the kit by Adaltis Inc., 2007. Testes were fixed in formaldehyde for histological analysis to be done. Values are means ± S.E.M. and compared by ANOVA. Significant decrease in serum testosterone was observed in the test 2 group. p<0.05. (control, 3.71±0.03 ng/ml; test 1, 3.67±0.03 ng/ml; test 2, 3.57±0.04 ng/ml). Body weight of each group before the study was compared with their weight after the study and the decrease observed was not significant. There were morphological changes in the testes of the garlic – fed rats. Compared to the controls, the test groups had marked degenerative changes in most of the seminiferous tubules with reduction in diameter, and the interstitial spaces were increased. Interstitial cells of Leydig cells were reduced in number. The exact mechanism(s) by which garlic affects male reproduction is unclear, but a decreased serum testosterone following treatment with crude garlic would decrease spermatogenesis. It has been hypothesized that garlic inhibits steroidogenesis in 3 different ways other than a decrease in its substrate income, viz: it might affect free cholesterol mobilization towards Leydig cell mitochondria, disrupt cholesterol mitochondria translocation, which is an important step in steroidogenesis with the steroidogenic acute regulatory (STAR) protein as an effector, and that garlic might prevent cholesterol conversion into testosterone by impairing key regulatory enzymes of steroidogenesis. Perhaps the decreased serum testosterone levels observed especially in test 2 rats may have been caused by any of these hypothesized mechanisms. Also, because the Leydig cells markedly degenerated and reduced in number especially in test 2 rats, the testosterone levels would also reduce. Therefore, garlic consumption especially at high doses or concentrations may cause a decrease in serum testosterone level and marked degenerative changes in testicular morphology.Key words: Garlic, serum testosterone, testes.