The relative importance of plasma osmolality (Posm) and left atrial pressure (LAP) as controllers of plasma vasopressin concentration (PAVP) was investigated in conscious dogs (n = 8). New techniques for the continuous measurement of LAP and computerized data collection permitted monitoring of LAP and mean arterial pressure (MAP) while Posm was suppressed with intravenous distilled water and then elevated with hypertonic NaCl. PAVP was determined by radioimmunoassay. This relationship between Posm and PAVP was determined at low, normal, and high (-0.9 +/- 0.8, 1.8 +/- 0.8, and 7.7 +/- 1.1 cmH2O, respectively) levels of LAP produced by either hemorrhage or autotransfusion of whole blood. MAP was not altered by these maneuvers. The results of these short-term studies have demonstrated a significant modulation of the osmoregulation of PAVP by changes in blood volume as indexed by LAP. Because of this modulation, PAVP is regulated so that contraction or expansion of the blood volume is more expeditiously corrected than would occur if osmoreceptors alone regulated plasma vasopressin.