Plasma and pituitary gonadotropin (GtH) levels, and the gonadotropin releasing hormone (GnRH) in differents areas of the brain, were measured using radioimmunoassays during the prespawning and the spawning period in a wild population of roach (Rutilus rutilus). GtH pituitary levels did not vary. On the contrary GtH plasma levels showed variations during the prespawning period and were minimum on the day of arriva1 on the spawning areas, Do. During spawning they increased in ovulated females but also in unovulated animals and returned to low values, 1 month after. During this period GnRH varied inversely in all brain structures and was always negatively correlated with the blood plasma GtH levels. This suggests that all the brain GnRH would be involved in the control of the GtH secretion during spawning. This was not the case during the prespawning period, during which only hypothalamic and pituitary GnRH levels were correlated with GtH. There were also marked variations in the telencephalic GnRH extra hypothalamic GnRH. It is hypothetized that it could be a link between environmental factors and the hypothalamo-pituitary cornplex.