The effects of diurnal and nocturnal injections of various hormonal preparations on the dynamics of moult and ovulation rate have been compared in hens kept under the conditions of a shortened (6-hour) illumination period. It was found that ACTH, pituitrin and arginine-vasotocin inhibited the moult if they were injected during night time. Prolactin and diethylstilbestrol enhanced the moult only after nocturnal injections, whereas diurnal injections of these hormones did not affect the shedding. Nocturnal injections of deoxycorticosterone were twice effective with respect to moulting rate as compared with diurnal injections. Thyroxine stimulated the moult irrespectively of the time of injection. Nocturnal injections of ACTH caused retardation of the recovery of egg laying in hens. Nocturnal administrations of arginin-vasotocine enhanced the recovery of ovulation. Diurnal injections of ACTH and vasotocine did not affect the egg laying not capacity of hens. Reproductive function was inhibited by prolactine irrespectively of the period of its administration. The data obtained indicate the existence of diurnal changes in the sensitivity of target tissues of hens to some of the hormones. At night time this sensitivity is usually increased.