The possible nitrate-induced heparin resistance was studied intraoperatively in 40 patients undergoing coronary artery bypass grafting. The patients were randomized to receive a continuous infusion of placebo, nitroglycerin (0.5 microgram kg-1 min-1) or isosorbide dinitrate (0.5 or 2.5 micrograms kg-1 min-1). After the infusion had been administered, prior to the institution of cardiopulmonary bypass, for at least 60 min, porcine intestine heparin 300 I.U. kg-1 (as divided in two consecutive doses of 100 and 200 I.U. kg-1, respectively) was administered to achieve systemic anticoagulation. Activated coagulation time values and plasma heparin anti-X, activity showed no significant differences between the groups before and after the administration of heparin. It is concluded that in doses given in the present study, organic nitrates do not interfere with the anticoagulation effect of large doses of heparin required for the conduction of cardiopulmonary bypass.