1. 1. Seasonal variation in plasma androgen levels was determined for males of the lizard Sceloporus undulatus. 2. 2. Plasma androgens peaked in both spring and in fall prior to brumation, coinciding with periods of maximum testicular enlargement and advanced spermatogenic development. 3. 3. Plasma androgen concentrations were minimal during summer, following testicular regression and breeding in spring. 4. 4. Epididymal masses were positively correlated to plasma androgen levels, while abdominal fat body masses demonstrated a negative correlation. © 1985.