Plasma atrial natriuretic factor concentrations in Rhode Island red hens averaged 72.1 +/- 6.9 pg.ml-1, range 33.4-136.0 pg.ml-1. The intravenous infusion of isotonic saline containing 3% dextran for 2 h produced no significant changes in plasma osmotic or electrolyte concentrations; however, heamatocrit changes indicated vascular expansions of 14.4% after 1 h and 21.3% after 2 h and plasma atrial natriuretic factor concentrations were elevated by 190% and 257% respectively. The intravenous infusion of chicken atrial nutriuretic factor at rates of 10, 25, 50 and 100 ng.kg-1.min-1 for 20 min produced levels of plasma atrial nutriuretic factor that were directly related to the infusion rate and which, in birds undergoing a steady-state diuresis/natriuresis driven by the intravenous infusion of isotonic saline at 1 ml.min-1, produced dose-dependent increases of 19, 26, 38 and 55% in urine flow rate and of 8, 30, 49 and 77% in sodium excretion. Potassium excretion was significantly increased only at the two highest atrial natriuretic factor infusion rates. The observed correlation between plasma atrial natriuretic factor concentration and vascular volume together with the atrial natriuretic factor-induced modulation of renal salt and water elimination is consistent with the concept that in the chicken this peptide has a physiological role as a regulatory hormone in volume homeostasis.