The spontaneous firing activity of olfactory bulb mitral cells and their response to intrabulbar infusion of GABAA receptor antagonist bicuculline were studied in ovariectomized, urethane-anesthetized female rats. Mitral cells recorded in the absence of specific stimuli and nasal airflow displayed three distinct patterns of spontaneous firing: high-frequency bursts with relatively long silent periods; high-frequency bursts without accurately defined silent periods; continuous. Infusion of bicuculline (0.2 nmol) into the bulb yielded inconsistent results on spontaneous firing and its inhibition evoked by electrical stimulation of the lateral olfactory tract. Of 32 cells tested, the spontaneous firing rate increased for 24, decreased for 6, and was not altered for 2. Likewise, of 18 cells tested, the poststimulus inhibitory period shortened for 12, lengthened for 4, and not altered for 2. An increased dose (0.4 nmol) of bicuculline produced dose-response relationships simply without any reversal effect. These results suggest that the heterogeneity of mitral cells exists in the context of local interneuronal circuitry in the bulb.