Two trials were conducted to determine the effect of dietary molybdenum (Mo) on the fertility of young rams, when fed for ca. 14 weeks to prevent copper (Cu) toxicity. In the first trial, the sexual development of young weaner rams receiving 38 mg Mo/kg DM (Mo group) was compared to that of a control group receiving no molybdenum. Both groups reached puberty at the same body mass. Although the mass of the testes of the rams in the control group was lower (P < 0,05) than that of the Mo group at slaughter, testicular mass, expressed as a percentage of warm carcass mass, did not differ between treatments. The control group showed a lower degree of spermatogenesis, expressed as the percentage tubules in the testes with spermatozoa, than rams fed molybdenum. This was assumed to be due to the lower body and testicular mass of the control group at slaughter. No histological evidence of testicular degeneration was observed in either group. In the second trial, young sexually-mature rams were fed a diet in which Mo concentration increased from 20 to 65 mg/kg DM during the experimental period. Sulphur(5.4 g/kg DM, Mo + S group) was included in the diet of one group, while the other group received no additional sulphur (Mo group). Although the Mo concentration in the testes of the Mo group was higher (P < 0,01) than in that of the Mo + S group, no differences between treatments were observed in testicular mass or in any of the measurements of fertility in the rams.