During an antiorthostatic posture change, left atrial (LA) diameter and arterial pulse pressure (PP) increase, and plasma arginine vasopressin (AVP) is suppressed. By comparing the effects of a 15-min posture change from seated to supine with those of 15-min seated negative pressure breathing in eight healthy males, we tested the hypothesis that with similar increases in LA diameter, suppression of AVP release is dependent on the degree of increase in PP. LA diameter increased similarly during the posture change and negative pressure breathing (9 to 24 mmHg) from between 30 and 31 ± 1 to 34 ± 1 mm (P < 0.05). The increase in PP from 38 ± 2 to 44 ± 2 mmHg (P < 0.05) was sustained during the posture change but only increased during the initial 5 min of negative pressure breathing from 36 ± 3 to 42 ± 3 mmHg (P < 0.05). Aortic transmural pressure decreased during the posture change and increased during negative pressure breathing. Plasma AVP was suppressed to a lower value during the posture change (from 1.5 ± 0.3 to 1.2 ± 0.2 pg/ml, P < 0.05) than during negative pressure breathing (from 1.5 ± 0.3 to 1.4 ± 0.3 pg/ml). Plasma norepinephrine was decreased similarly during the posture change and negative pressure breathing compared with seated control. In conclusion, the results are in compliance with the hypothesis that during maneuvers with similar cardiac distension, suppression of AVP release is dependent on the increase in PP and, furthermore, probably unaffected by static aortic baroreceptor stimulation.