1. The effects of different photoperiods on the concentrations of plasma androgens, testes weight and on the diurnal rhythm of plasma testosterone (T), dihydrotestosterone (DHT) and androstenedione (A) in mature, single comb White Leghorn male chickens were studied. 2. Birds were exposed to either 14 hr of light (lights on at 0600-2000 hr) or to 24 hr of light per day. 3. Blood samples were collected from birds in both groups at 3-hr intervals and plasma levels of T, DHT and A were measured using radioimmunoassays. Following blood collection, birds were weighed, killed and testis weights were recorded. 4. Under 14 hr of light, there was a diurnal rhythm of T and DHT with hormone concentrations peaking at the end of the dark period. There was no obvious rhythm for A. Exposure to 24 hr of light abolished the diurnal rhythm found under 14 hr of light. 5. There was an increase not only in testis weights but also in body weight and hormone concentrations under 24 hr of light. 6. It was concluded that photoperiod plays an important role in controlling the concentration and rhythm of androgens in mature male chickens.