Transmission in the cardiac vagal reflex pathway can be inhibited by stimulation of the hypothalamic defense region or somatic afferent nerves. A pharmacological analysis of inhibitory modulation of reflex vagal bradycardia was undertaken in the present study. Picrotoxin (0.5--1.5 mg/kg i.v.) or bicuculline (0.5--1.5 mg/kg i.v.) produced a dose-related blockade of inhibition of reflex vagal bradycardia elicited by stimulation of the lateral hypothalamus or branches of the brachial plexus in spinal (C1 or C8 transected) cats. In contrast, strychnine and pentylenetetrazol failed to change the heart rate responses produced by stimulation of the hypothalamus or brachial plexus afferents. Picrotoxin and bicuculline also blocked inhibition of reflex vagal bradycardia produced by stimulation of the inferior olive in decerebrate spinal cats. This observation supports the contention that these agents act in the brain stem to block inhibitory modulation of reflex vagal bradycardia. In addition, picrotoxin and bicuculline lowered basal heart rate in spinal cats but not in decerebrate spinal cats. This observation suggests that tonic suprabulbar inhibition of reflex vagal bradycardia also is sensitive to blockade by picrotoxin and bicuculline.