Male Djungarian hamsters were raised from birth in either long (16L:8D) or short (8L:16D) photoperiods. In 16L:8D, testes were large (about 400 mg) and showed full spermatogenesis from 31 days of age, the cauda epididymidis contained motile spermatozoa from 35 days and the accessory glands were large and reached their final size at about 60 days of age. In 8L:16D, testes remained small (about 15 mg) and undeveloped up to an age of about 130 days; they then developed to reach normal size and full spermatogenesis. In 16L:8D body weight increased steadily up to an age of about 160 days, but in 8L:16D body weight increase was slower and reached normal values, after a plateua at about 80 days, at about 190--260 days. Most animals in 8L:16D moulted into winter pelage, and then into the summer pelage; those in 16L:8D kept the summer pelage throughout.