Thirty-two lambs (8/breed) were used for puberty determination in 4 breeds, between 5 to 14 months oid. The first appearance ofspermatozoa in the ejaculate occurred at weeks 26.5,26.0,27.5, and 25 of age when the lambs had attained a body weight of 28.3,27.2,24.7 and 27.2 Kg, and a scrotal circumference of 22.4,22.8, 19.4 and 22.9 cm for Mora Colombian, Romney Marsh, Criolla and Hampshire breeds respectively. Puberty was defined at 36 weeks, when ejaculate reached: sperm concentration >1500 x 10 Spzs/ml, and >60% forsperm traits remaining. There were no significan! differences in the age at puberty between breeds, but there were significant differences in body weight and scrotal circumference (P<0.05). The second elevation of testosterone levels, at 32 weeks of age (0.78 ng/ml), coincides with the attainment of puberty.