The activated clotting time (ACT) has been used clinically for 41/2 years to monitor the adequacy of intraoperative heparinization in both cardiopulmonary bypass (CPBP) and peripheral vascular surgery (PVS) patients. Since January, 1976, we have operated on 440 PVS patients in our institutions, of whom 255 had intraoperative monitoring of heparinization by means of the ACT test; an additional 185 patients received heparin according to an empiric protocol. An automated machine for determining the ACT (Hemochron) had been used on the last 28 patients in the ACT group. We conclude that: (1) the ACT is an acurate method of monitoring anticoagulation during PVS, (2) the Hemochron is a simple and effective machine to perform the ACT, (3) values obtained by the Hemochron generally exceed those of the ACT done by hand by approximately 20 seconds at lower levels of heparin and are equal at higher levels of heparin, (4) supplemental heparin was required in 22% of our patients to maintain their ACT values at twice control values, (5) the response to heparin is twofold: an initial sensitivity or resistance followed by an independent and variable rate of consumption, and (6) the ACT is a useful method for predicting inadequate heparin reversal or heparin rebound at the conclusion of the operation.