AbstractA study of plasma arginine vasopressin (AVP) levels and their relation to changes in arterial pressure was undertaken in 13 patients undergoing open heart surgery. Seven of the patients received high doses of intravenous morphine (1 or 2 mg/kg) as part of their anaesthetic, whereas the other six did not. Increases in the AVP levels during cardiopulmonary bypass (CPB) were significant in both groups, but were significantly greater in the patients not receiving morphine (median 86·1 fmol/ml) than in those who did (47·5 fmol/ml). In both groups there was a linear correlation between the natural logarithm of the plasma AVP level and the percentage fall in the mean arterial pressure, although the slope of the relationship was significantly less in the group receiving morphine than in the group not receiving morphine. These results indicate that in patients undergoing cardiac surgery the neuroendocrine reflex control of plasma AVP levels in relation to changes in arterial pressure is intact, but that the sensitivity of the reflex (i.e. the increase in plasma AVP level for a given fall in arterial pressure) is reduced by the administration of high doses of intravenous morphin