Prolonged thirst provokes an activation of the LHRH system which enables the visualization of fiber connections not seen in untreated control animals. This type of experimental stress situation increases the number of LHRH-containing fibers in the organum vasculosum laminae terminalis and in the median eminence. The number of LHRH-producing cells in the preoptic nucleus is increased and the fiber connection between this area and the median eminence can be observed. The tanycytes and the perikarya of the arcuate nucleus do not react with the antibody against LHRH. Moreover, during thirst, a network of LHRH-containing fibers is observed in the medial mammillary nucleus. The results obtained at the light microscopic level have been confirmed and supplemented by electron microscopic immunocytochemical observations.