Erythrocyte cyanide levels were determined by a sensitive fluorimetric method on four occasions during coronary bypass in hypothermia in 18 consecutive patients treated with sodium nitroprusside (SNP) with an infusion rate less than 1 microgram x kg-1 x min-1. Every second patient received the cyanide antidote thiosulphate simultaneously with the SNP-infusion. At normal body temperature, as well as during hypothermia in cases receiving thiosulphate, the cyanide levels rose slowly but significantly with the infusion rate. Higher erythrocyte cyanide levels in relation to the infusion rates, up to 8.0 mumol/l, were found during hypothermia in two of the cases not receiving thiosulphate. We conclude that SNP is broken down to cyanide even under hypothermia and that low body temperature may impair the conversion of cyanide to thiocyanate, probably by affecting the metabolic pathways providing the sulphur substrate. This effect may add to other factors decreasing sulphur availability in critically ill patients, and simultaneous administration of thiosulphate is therefore recommended to ensure a safe SNP treatment during and after coronary bypass operations.