ABSTRACTPulmonary arterial pressure (PAP)-flow coordinates were obtained in 14 anesthetized dogs before and after pulmonary hypertension was induced with autologous blood clots. Cardiac output (CO) was altered by systemic arteriovenous fistulas. The PAP-CO coordinates were always rectilinear.Before emboli, the mean vascular closing or outflow pressure (the pressure intercept of the PAP-CO line) was 8.8 2.1 (SD) mm Hg. Emboli increased PAP (15.1 + 1.6 to 36.5 + 3.5 mm Hg; p < .001) and decreased CO (3.8 + 0.6 to 2.4 + 0.8 liters min-'; p < .001). Incremental resistance (the slope of the PAP-CO line) only increased slightly. On the other hand, the marked increase in PAP was predominantly due to an increase in effective outflow pressure (from 8.8 + 2.1 to 28.6 ± 3.6; p < .001). Hydralazine was administered in a dose sufficient to double CO. This did not affect PAP and caused an inconsistent and small decrease in incremental resistance. However, a consistently significant decrease in effective outflow pressure, averaging 23%, was observed. In this canine preparation of pulmonary hypertension the predominant effect of hydralazine appears to be a decrease in the mean vascular closing or outflow pressure.