Some aspects of sexual development and mating behaviour in Clun Forest ram lambs having a wide range of birth dates were studied during two seasons. The 61 ram lambs studied were introduced individually to Clun ewes in oestrus, for short periods at fortnightly intervals during late summer and autumn. Complete matings, i.e. intromission and ejaculation, were first achieved at a mean age of 6 months (range 142–223 days) and a mean body weight of 35 kg (range 24–49 kg). The attainment of physiological puberty preceded the first copulation, often by several weeks. A few individuals had abnormal testes and were partly or completely sterile, but exhibited normal mating behaviour. Early-born lambs tended to complete their first matings at a higher age and heavier body weight than late-born lambs, which suggests a possible seasonal influence on the development of breeding activity in the ram lamb.