Atrial wall stretching is a known stimulus for atrial natriuretic peptide (ANP) secretion. The effects of the stimulation of autonomic nervous system, hemodynamic factors, and humoral factors (epinephrine, angiotensin, vasopressin, and brain extracts) on the release of ANP under basal conditions and during increased atrial pressure produced by acute volume loading in pithed rats were examined. In conscious rats, acute volume expansion by 0.9percent; of saline (4 ml) increased the plasma immunoreactive ANP (IR-ANP) concentrations by a factor of 4 (140±30 pg/ml vs. 521±140 pg/ml, p<0.001, n=8), whereas volume-induced ANP release was blocked in pithed rats (75±9 pg/ml vs. 99±13 pg/ml, NS, n=7). The ANP versus right atrial pressure curve shifted to the right, indicating that much smaller amounts of IR-ANP were released in pithed than in conscious rats for each given increase in right atrial pressure. Electrical vagal and sympathetic nerve stimulations or changes in heart rate had no effect on plasma IR-ANP concentrations and failed to restore the volume-load-induced release of ANP in pithed rats. When extracts of anterior pituitary lobe, brain cortex, or hypothalamus were infused, no effect on volume-expansion-induced plasma IR-ANP levels was seen. In contrast, acute volume expansion caused a fourfold increase in levels of circulating IR-ANP in pithed rats that received posterior pituitary extracts, and the ANP versus right atrial pressure curve shifted markedly to the left. Infusion of a V1antagonist blocked the volume-expansion-induced ANP release produced by the posterior pituitary extract. When [Arg8]-vasopressin (0.025 or 0.05 μg/kg/ min) was infused to pithed rats, mean arterial pressure increased but basal plasma IR-ANP did not change significantly. However, acute volume expansion in the presence of vasopressin infusion (0.05 μg/kg/min) increased the amount of circulating IR-ANP by a factor of 4 (113±14 pg/ml vs. 414±43 pg/ml, p<0.001, n=8). Thus, for a given increase in right atrial pressure, a similar amount of IR-ANP was released in the pithed rat during the vasopressin infusion as in the normal conscious animal. V1antagonist blocked the increase in mean arterial pressure as well as the increase of plasma IR-ANP produced by [Arg8]-vasopressin. In addition, volume expansion during intravenous epinephrine (1.75 μg/kg/min) and angiotensin (1.0 μg/kg/min) doubled plasma IR-ANP levels. These results indicate that pressor hormones, especially vasopressin, restore the ability of volume expansion to induce ANP release in the pithed rat. The combined effect of right atrial pressure and pressor hormones in regulating ANP release is a new mechanism by which humoral stimulation modulates the direct, mechanical-stimulus-induced hormone secretion.