The effect of arginine vasopressin (AVP) and phenylephrine (PE) infusions on mean arterial pressure (MAP) and heart rate (HR) were compared in conscious dogs with all autonomic receptors intact (I), during muscarinic blockade (MB) and during ganglionic blockade (GB). After either MB or GB, the dose-MAP response curve for AVP and PE was shifted to the left of the I response curve; a greater shift was observed with AVP than with PE. The MAP threshold after GB for AVP and PE occurred at 10 and 50% of the threshold dose observed during the I response, respectively. Not only did the MAP threshold occur at a lower dose after MB and GB, but also the slope of the response curve was steeper than that of the I response. Comparing the amount of drug necessary to increase MAP 25 mmHg above control for PE and AVP before and after GB, the intact PE response required 4.3 +/- 1.0 (P less than 0.01) times more drug than during GB versus the intact AVP required 16.8 +/- 2.8 (P less than 0.01) times more drug than during GB. The baroreflex control of HR when all receptors were intact was 3.4 +/- 0.4 (P = 0.001) times more sensitive during AVP compared with PE; no differences were observed after MB. There were no significant changes in HR to AVP or PE after GB, thus indicating a lack of a direct effect of these agents on the HR. Our results show that MB and GB equally potentiate the pressor effects of AVP and PE, and the augmentation was much greater for AVP than for PE. The difference in the potentiation of these two vasoconstrictors is consistent with the finding that the baroreflex sensitivity during AVP was enhanced compared with PE. We have postulated that, in the resting conscious dog, AVP increases the sensitivity of the baroreflex primarily by producing a greater level of parasympathetic tone to the heart in response to a given pressure stimulus.