1. The response to arginine vasopressin (AVP) at doses of 5 and 10 pmol (100 g body weight)-1 h-1 was studied in conscious rats during the infusion of 1% (w/v) dextrose at 11.6 ml h-1 with and without pre-treatment with indomethacin. 2. In the absence of indomethacin AVP infusion induced dose-related increases in sodium output that were positively correlated with increases in mean arterial blood pressure (MAP) and plasma atrial natriuretic factor (ANF) immunoreactivity. Increases in renal prostaglandin E2 (PGE2) synthesis were also associated with AVP infusion. 3. Indomethacin pre-treatment abolished the AVP-induced increases in renal PGE2 synthesis and also the dose-related differences in ANF immunoreactivity. Increases in MAP and sodium output were unaffected at the 10 pmol (100 g body weight)-1 h-1 dose of AVP and only slightly attenuated for the 5 pmol (100 g body weight)-1 h-1 dose. 4. For both series AVP induced marked falls in glomerular filtration rate (GFR) but only small transient falls in effective renal plasma flow. The observed falls in GFR support the view that the natriuresis is due to changes in tubular handling and not in the filtered load of sodium. 5. It is concluded that the natriuresis elicited by AVP is closely related to the pressor action of the hormone but renal PGE2 synthesis and plasma ANF are not responsible for mediating this response.