SUMMARY1. To determine the long‐term haemodynamic and humoral effects of arginine vasopressin (AVP), a chronic intravenous infusion of AVP was performed in conscious Wistar normotensive rats.2. AVP (1, 10, 50 or 100 ng/h) or saline as a vehicle control was infused continuously into the right jugular vein at a rate of 1 μL/h using an osmotic minipump for 7 days.3. As a result, significant elevations of systolic blood pressure were observed in association with increases in plasma AVP concentration. Significant decreases in heart rate were observed during infusion of 100 ng/h of AVP. Mean arterial pressures measured directly on the sixth day of infusion were significantly higher in the rats given 50 ng/h (125 ± 3 mmHg) or 100 ng/h (125 ± 2 mmHg) compared with control rats (117 ± 2 mmHg). Intravenous injection of the V1antagonist, d(CH2)5Tyr(Me)AVP, significantly reduced the elevated mean arterial pressure induced by 50 or 100 ng/h of AVP (—7 ± 4 and—11 ± 2 mmHg, respectively). Plasma renin and norepinephrine concentrations were not affected by AVP infusion, while plasma epinephrine concentration was lower in the rats given 100 ng/h of AVP. Intravenous infusion of AVP did not alter bodyweight, serum electrolytes or osmolality.4. These results suggest that AVP has a long‐term pressor effect which is attributable to its vasoconstrictor action in c