AbstractMale European starlings (Sturnus vulgaris) were held in constant darkness for 448 days. Testicular widths, measuredin situvia unilateral laparotomy at two to four week intervals, revealed that testicular growth reached a size associated with spermatogenesis in an average of 101 days. The spermatogenic period persisted for an average of 296 days. Histological examination of testes from two birds after 280 days of treatment revealed mature sperm in the seminiferous tubules. Complete testicular involution did not occur. Current photoperiodic theory must be modified to explain testicular metamorphosis without daily photostimulation in an avian species known to be photoperiodic.