The testicular condition of adult and juvenile Mallard drakes has been studied by exploratory laparotomy after treatment with different daily photoperiods. Juveniles reared from hatching under “long” (20L: 4D) photoperiods slowly adopt adult male plumage and become sexually mature. However, treatment with “short” (8L: 16D) photoperiods from days 21–103 of age hastens the appearance of adult plumage and increases the reproductive response when subsequently returned to long photoperiods. Adult drakes will undergo a cycle of testicular maturation and regression on long photoperiods but, in contrast to other species, do not remain sexually quiescent indefinitely while treatment with long photoperiods is continued. Some degree of similarity between juvenile and adult sexual quiescence in Mallard drakes is suggested by the data.