A radioimmunoassay was set up to measure plasma arginine vasopressin levels (AVP). Characteristics of the assay include a sensitivity to 0.25 pg, high specificity of the antibody, mean recovery of added unlabeled arginine vasopressin of 80%, and an interassay coefficient of variation of 7.6%. This assay was used to investigate the influence of various factors on plasma vasopressin levels in a total of 121 awake male rats. Blood samples obtained in Wistar rats via an indwelling arterial catheter yielded results similar to those following decapitation, i.e., 1.27 +/- 0.26 vs. 1.68 +/- 0.39 pg/ml. Forty-eight hour dehydration markedly increased plasma AVP to 21.8 +/- 2.39 pg/ml. AVP was less than 0.5 pg/ml in Brattleboro rats. Low and high sodium intake, angiotensin II infusion (10 and 30 ng/min), and converting enzyme inhibition by captopril (100 mg/kg) did not alter plasma AVP. During norepinephrine infusion (250 ng/min) plasma AVP rose to 5.28 +/- 1.29 pg/ml, whereas it tended to fall with isoproterenol infusion (10 ng/min). Plasma AVP was slightly higher in spontaneously hypertensive rats than in Wistar-Kyoto controls.