Previous investigations have suggested that the duration of the hangout interval is determined by long-term alterations in right ventricular systolic loading rather than the length of right ventricular systole. The factor(s) that effect short-term changes in this interval are unknown. Accordingly, we altered pulmonary artery pressure with phenylephrine and nitroprusside to achieve three peak pressures at low, medium, and high levels in each of 10 normal patients. Simultaneous biplane right ventricular volumes, high-fidelity pulmonary artery and right ventricular pressures, and pulmonary artery flow/velocity were obtained at each condition. After pharmacologic autonomic blockade (atropine and propranolol) at a constant heart rate achieved with atrial pacing (93 beats/min), peak pulmonary artery pressures differed significantly at low, medium, and high loading conditions (23 ± 3 vs 31 ± 7 vs 42 ± 6 mm Hg, p < 0.01). No associated significant change occurred in the preejection period (109 ± 24 vs 98 ± 14 vs 106 ± 21 msec) and right ventricular ejection time (298 ± 24 vs 318 ± 23 vs 316 ± 24 msec). The hangout interval differed significantly at each loading condition (43 ± 16 vs 34 ± 13 vs 23 ± 6 msec, p < 0.01) and correlated inversely with both the peak pulmonary artery pressure (r = -0.53, p < 0.01) and the right ventricular end-systolic pressure (r = -0.56, p < 0.01). In contrast, the hangout interval and right ventricular ejection time were not linearly related. Therefore we conclude that the hangout interval is determined short-term by the level of pulmonary vascular loading rather than the duration of right ventricular systole. © 1988.