Abstract Exposure of proestrous rats in the afternoon (1700h) or night (0100h) to intact and castrated male pheromones induced changes in olfactory bulbs and hypothalamic glutamic acid decarboxylase activity. Compared to the values found in control rats, the enzyme activity in the main olfactory bulb measured in the afternoon did not discriminate between the odor of intact or castrated males in the first hour of exposure, whereas in the accessory olfactory bulb, the enzyme activity was increased specifically by castrated male odor. At night, main bulb-enzyme activity was reduced by the intact male pheromone 1 h later, whereas accessory bulb-enzyme activity was increased by castrated male odor. Preoptic area enzyme activity was only modified by castrated male odor in the afternoon, whereas intact male pheromone exposure for 2 h decreased enzyme activity specifically at night. In the medial basal hypothalamus, both olfactory stimuli decreased afternoon enzyme activity, and on the contrary no changes were seen by nighttime. It is conclude that, in proestrous rats, the activity changes of main bulb and preoptic area GABA-ergic system evoked by intact male pheromone may be related to a facilitatory neural mechanism involved in the rat sexual motivation during the night.