In spontaneously hypertensive rats (SHR) and their normotensive Wistar-Kyoto controls (WKY), prolonged intravenous infusions of either arginine vasopressin (AVP, 8 mU X kg-1 X min-1) or phenylephrine (PE, 20 nmol X kg-1 X min-1) resulted in similar rises in arterial pressure. Heart rate fell greatly in the WKY but not in the SHR. Withdrawal of the PE infusion resulted in moderate decreases in blood pressure and increases in heart rate; these responses were similar in SHR and WKY. At 5 h after PE withdrawal, blood pressure and heart rate returned to basal values. In contrast, withdrawal of the AVP infusion was associated with greater falls in blood pressure and rises in heart rate. Blood pressure and heart rate in both the SHR and the WKY at 5 h after AVP were significantly different from their respective basal values. The effects of AVP withdrawal on either blood pressure or heart rate were significantly greater in the SHR than in the WKY. At 5 h after the withdrawal of AVP, blood pressure in the SHR was reduced to normotensive levels. These results suggest that the withdrawal effect was specific to AVP, was more marked in the SHR, and might not result from only the rise in blood pressure seen during the intravenous infusion of the pressor agent.