The effect of morphine, diazepam, N2O/O2anesthesia on baroreflex control of heart rate in humans was investigated in this study. Group 1 subjects (n = 11) received morphine 0.5 mg/kg, diazepam 0.25 mg/kg, and 70% N2O with O2. Group 2 subjects (n = 10) received morphine 0.75 mg/kg, diazepam 0.25 mg/kg, and 70% N2O with O2. Phenylephrine (the pressor test), sodium nitroprusside (the depressor test), and graded neck suction were employed to alter the stimulation of baroreceptor sites. The pressor, the depressor, and neck suction baroreflex slopes declined significantly in both groups from awake to anesthetized. There was no significant difference in the degree of depression between the two groups for all three tests. Neck suction derived slopes compared favorably to the pressor test slopes (r = 0.70,P< 0.01). This study indicates that the depression of arterial baroreflex–heart rate responses under morphine, diazepam, N2O/O2anesthesia is similar to that seen with potent inhalational anesthetics such as isoflurane. Furthermore, there was no difference between the two morphine doses that were studied.