Female brown trout were sacrificed every month or twice a month during the reproductive season. Plasma gonadotropin (GtH) and estradiol-17 beta (E2-17 beta) were measured using radioimmunological techniques, in relation with the state of gametogenesis. From ovulation to July egg's diameters remained less than 1 mm, and ovogenesis was characterized from the histological appearance of three types of vitellus: glycoproteique (type I), lipidique (type II), lipidoprotéique (type III), and the immunological plasma detection of the vitellogenin. During this period GtH levels remained lower than 1 ng/ml except in March, when they increased around 5 ng/ml, both with pituitary GtH and plasma E2 17 beta. This rise occurred just before the appearance of the type III vitellus within the oocyte, and might correspond to a critical phase of the reproductive cycle during which vitellogenin could not be detected in 70% of the animals. During rapid growth of the oocyte from 1 to 5 mm, mean GtH levels increased from 0.75 to 2.5 ng/ml when those of the E2-17 beta increased more rapidly but began to drop before the end of vitellogenesis. Maturation and ovulation were accompanied by a rise of the GtH level, in correlation with the lower levels of E2-17 beta measured during the cycle, GtH remained high even after ovulation, and more in fish which had kept their eggs within the body cavity. Correlation among GtH, E2-17 beta, and the diameters of eggs had been calculated. There was a positive correlation among GtH, E2-17 beta levels, and the diameters of eggs during exogenous vitellogenesis, and a negative between GtH and E2-17 beta at the end of the reproductive cycle. These results were discussed, in relation to the existence of a pulsatile mode of GtH secretion.