To our knowledge the effect of androgen administration on testes of hypophysectomized birds has been studied in the pigeon (Columba livia) only. Chu (1940) and Chu & You (1940) reported that testosterone prevented hypophysectomy-induced testicular regression and that it restored the testicular weights if administered after the regression had occurred. We report here the results obtained in a study with Japanese quail (Coturnix c. japonica), an animal which is used frequently in laboratory experiments. Male Japanese quail, all hatched from eggs obtained from the same breeding pen, were kept in chick batteries in a room in which natural daylight was supplemented by 14 hr. (05.00 to 19.00 hr.)of fluorescent light. The temperature in the room was not controlled. The birds received a commercial quail breeder ration and water ad libitum. The birds were hypophysectomized as described elsewhere (Baylé, 1968), when the animals were 5–6 weeks old. Laparotomy on five successive