The peripheral vascular effects of isoflurane and halothane were compared in five critically ill patients in whom a Jarvik-7 artificial heart had been implanted. The lungs of all patients were mechanically ventilated in the postoperative period and the patients were monitored with an arterial catheter and with catheters that had been surgically inserted into the right and left atria and into the pulmonary artery. Noreperinephrine and epinephrine plasma concentrations were measured using a radioenzymatic assay. The Jarvik-7 settings were modified to render the artificial heart “preload independent” and to maintain cardiac output constant. Each patient was anesthetized twice using halothane and isoflurane at two different MAC levels, 1 and 1.5 (Datex vapour analyzer), with the interval between each anesthetic ranging from 12 to 26 h. Both anesthetics significantly decreased mean arterial pressure (from 100 ± 11 mmHg to 66 ± 13 mmHg for halothane and from 102 ± 17 mmHg to 48 ± 11 mmHg for isoflurane; mean ± SD) and systemic vascular resistance index (from 27 ± 11 Wood units/m2to 18 ± 6 Wood units/m2for halothane and from 30 ± 6 Wood units/m2to 13 ± 3 Wood units/m2for isoflurane; mean ± SD), but with isoflurane to a significantly greater extent than with halothane (P< 0.01). Both anesthetics induced significant and comparable decreases in right atrial pressure (from 15 ± 2 mmHg to 13 ± 4 mmHg for halothane and from 16 ± 3 mmHg to 11 ± 2 mmHg for isoflurane; mean ± SD), mean pulmonary arterial pressure (from 22 ± 10 mmHg to 17 ± 11 mmHg for halothane and from 19 ± 9 mmHg to 14 ± 8 mmHg for isoflurane; mean ± SD), and left atrial pressure (from 14 ± 6 mmHg to 11 ± 7 mmHg for halothane and from 11 ± 5 mmHg to 6 ± 5 mmHg for isoflurane, mean ± SD) without changing catecholamine plasma concentrations (from 474 ± 586 pg/ml to 206 ± 390 pg/ml for norepinephrine concentrations and from 100 ± 172 pg/ml to 96 ± 71 pg/ml for epinephrine concentrations during halothane, from 356 ± 92 pg/ml to 512 ± 269 pg/ml for norepinephrine concentrations and from 121 ± 118 pg/ml to 129 ± 95 pg/ml for epinephrine concentrations during isoflurane; mean ± SD). Because cardiac output was maintained constant throughout the anesthetic, these results suggest that halothane and isoflurane reduced vascular tone of veins, pulmonary vessels, and systemic arteries.