The Syrian golden hamster exhibits territorial aggression in a laboratory setting, attacking much more when in the home cage than in the cage of another hamster. In expt. 1 it was found that female hamster vaginal discharge (FHVD) greatly inhibits the territorial aggression of males. In expt. 2 it was found that hamster territorial aggression is almost completely eliminated by bilateral olfactory bulbectomy but is unaffected by unilateral olfactory bulbectomy. However, bilateral olfactory bulbectomized (BOB) hamsters fought back normally if attacked first. Blinding increased the territorial aggression of male hamsters. In expt. 3 a peripheral means of impairing olfaction was used. The results suggest that it is the olfactory impairment, rather than the loss of some hypothetical non-sensory functions of the olfactory bulbs, that causes the elimination of territorial aggression after olfactory bulbectomy. Because of the extensive secondary and tertiary olfactory projections to the limbic forebrain and hypothalamus, and because most laboratory mammals are macro-osmatic, the nature and degree of olfactory control should be an important consideration in studies on the neural basis of social behavior.