We have earlier demonstrated that quinine (QU) is a testicular toxicant. This present study was aimed at evaluating the effects of QU on both the serum and testicular levels of testosterone (TT) in an attempt to elucidate one of the potential mechanisms of QU-induced testicular toxicity. Thirty adult male Sprague-Dawley rats weighing 180-200g were used and were randomly divided into 3 groups of 10 rats each. Rats in group 1 had distilled water. Rats in group 2 had QU only at the dose of 10 mg/kg body weight per day (5 days in a week) for 8 weeks. Rats in group 3 rats had 10 mg/kg of QU (5 days in week) and 0.05 mg/kg of TT (3 days in a week) for 8 weeks. All the animals were sacrificed at the end of 8 weeks by decapitation. Seminal analysis was done on the tubular fluid aspirated from the caudal epididymides. The two testes were excised, weighed, and volume estimated. One testis of each rat (0.25 g of tissue) was homogenized with Potassium Chloride and TT level determined in the supernatant of the homogenate, while the other testis was processed for histology. Morphometry was carried out by assessing the diameter, cross-sectional area, number of profiles per unit area, length density and numerical density of the seminiferous tubules, and the relative and absolute volume of testicular components. The serum levels of TT in all the animals were also determined at the time of sacrifice. Both the serum and testicular levels of TT in rats administered QU only were significantly (P < 0.001) lower than those of the control and QU plus TT-treated rats. We conclude that QU induces spermatogenic epithelial toxicity by possibly interfering with the steroidogenic function of the Leydig cell.