1. Immunoreactive androgens were measured in the urine of individual male tree-shrews throughout post-natal development. 2. Urinary androgens were low during the infantile phase and then rose significantly between the 36-45 and 46-55 day age groups, in association with the onset of pubertal development. 3. Androgen excretion increased linearly during the pubertal period in parallel with the progressive development of the testes and reproductive tract. 4. The precise endocrine correlates established in the present work suggest that serial determination of urinary androgens provides a reliable method of monitoring male reproductive development which may be applied to small, sensitive species.