Daily changes in carp gonadotropin levels in adult female carp and daily changes in carp oocyte sensitivity to carp hypophysial homogenate, in vitro and in vivo, were investigated. A total of three series of experiments were carried out. Gonadotropin levels were radioimmunologically determined. The results of series 1 and 2 experiments were subjected to statistical analysis with the use of cosinors circle and ellipse of errors. It has been found that in the mature female carp in the pre-spawning period with the light periods being long (L:D = 16:8) the apogee for gonadotropin occurs 10 hr after the onset of the light period. The sensitivity of the oocytes, in terms of the percentage of mature oocytes (after GVBD) following a 24-hr incubation of ovarian fragments with the hypophysial homogenate, reached the highest value at 1300, i.e. 9 hr after the onset of the light period. It was also found that the injections of carp hypophysial homogenate made at 0900 were much more efficient in inducing ovulation than those at 2100.