Angiotensin II (AII) was infused into the lateral cerebral ventricle of rats given water, isotonic saline, or hypertonic saline ad libitum, or 40 ml water/day. Fluid intake, change in body weight, plasma [Na+], and plasma and pituitary arginine vasopressin (AVP) levels were measured. Isotonic saline or All (1 µg/µl saline) was infused at 1 µl/h for 5 days using osmotic minipumps. All increased fluid intake of rats given isotonic saline to drink; they consumed an average of 269 ± 25 ml/day on day 5. All infusions in rats given water or isotonic saline to drink decreased plasma [Na+] with no changes in plasma or pituitary AVP. However, in rats given hypertonic saline, plasma [Na+] remained at control levels while plasma AVP increased. In water-restricted rats, the effects of AII were intermediate: a small decrease in plasma [Na+] and a small increase in plasma AVP. From these results, it is suggested that although acute All administration elicits AVP release, this effect diminishes during chronic ATI infusion, coincident with reduced plasma [Na+