An afterdrop in core temperature after hypothermic cardiopulmonary bypass (CPB) is related to inadequate peripheral rewarming. We proposed that pharmacological vasodilatation during rewarming on bypass would improve peripheral rewarming and reduce the degree of afterdrop. Fifty-nine of 120 patients were randomized to receive a sodium nitroprusside (SNP) infusion during the rewarming stage of hypothermic CPB. Mean systemic vascular resistance (SVR) during the rewarming phase of CPB was 1129 dyne s-1 cm-5 in the control group and 768 dyne s-1 m-5 in the SNP group (P < or = 0.001). Patients receiving SNP rewarmed to 37.0 degrees C faster (299 min vs 376 min; P = 0.003) and were extubated earlier (490 min vs 621 min; P = 0.001). Patients receiving SNP had a warmer mean peripheral temperature (MPT) (32.9 degrees C vs 32.4 degrees C; P = 0.05) on termination of CPB. Postoperative core temperature fell less in the SNP group (35.6 degrees C vs 35.2 degrees C; P = 0.01) as did MPT (31.8 degrees C vs 31.2 degrees C; P = 0.004). SNP-induced vasodilatation during rewarming from hypothermic CPB improves peripheral rewarming, reduces the degree of postoperative core and peripheral hypothermia and reduces time to extubation.