Increased mean arterial pressure during the aortic crossclamp period while on cardiopulmonary bypass was usually treated by us with hypotensive drugs. We noticed, however that aspirating shed excess pulmonary venous blood from the open pleural cavities causes an immediate reduction in mean arterial pressure, obviating the need for any further pharmaceutical intervention. In this study we investigated the relationship between the reduction in mean arterial pressure and the levels of prostacyclin and prostaglandin E2 in the peripheral and pulmonary venous blood. Ten men undergoing coronary bypass operations had 21 episodes of increased mean arterial pressure (106.9 ± 11.4 mmHg) during aortic crossclamping, which was reduced to 67.4 ± 11.4 mm Hg (p < 0.001) only by aspirating a mean of 490 ml (range 150 to 1100 ml) of pulmonary venous blood from the pleurae back into the circulation. Mean peripheral prostacyclin level, measured as 6-keto-prostaglandin F(1α), and prostaglandin E2 level, both measured by radioimmunoassay technique, were significantly lower at peak mean arterial pressure (419 ± 180 and 59.5 ± 21.2 pg/ml) than at lowest mean arterial pressure (632 ± 271 and 96.7 ± 52.4 pg/ml for 6-keto-prostaglandin F(1α) and prostaglandin E2, respectively; p < 0.001). Prostaglandin F(1α) and prostaglandin E2 levels in the aspirated pulmonary venous blood were 2309 ±3098 pg/ml and 749 ± 909 pg/ml, respectively. The hypotensive effect of shed pulmonary venous blood that is aspirated back from the pleurae into the circulation seems to be mediated by the high levels of prostacyclin and prostaglandin E2, both powerful vasodilators.