Cardiovascular effects of N2O during enflurance anesthesia were studied in 12 healthy, young volunteer subjects ventilated to maintain normal PaCO2 [pulmonary arterial pressure of CO2]. Twelve circulatory variables were measured and 13 more calculated. When N, 70%, was added to enflurane, 1.86% (1 MAC [minimum aveolar concentration]), or enflurane, 2.93% end-tidal, no change was observed. When N2O, 70%, was added, only minimal changes were observed. In a 2nd part of the study, enflurance was compared with enflurane-N2O, 70%, at equipotent levels. The following 3 variables (in percentages) decreased less in relation to awake control values at 1 MAC enflurane-N2O oxide-O2 than at 1 MAC enflurane-oxygen: left ventricular stroke work, -47.2 vs. -55.9; aortic dP/dt [changes in pressure with time], -44.0 vs. -57.1; pressure-pulse product, -26.6 vs. -39.4. Forearm venous compliance decreased more: -26.0 vs. 2.9. The difference between the anesthetic mixture was much more noticeable at 1.5 MAC, where eight variables (in percentages) decreased less with enflurane-N2O-O2 than with enflurane-O2: cardiac output, -6.9 vs. -22.1; stroke volume, -31.4 vs. -46.0; left ventricular minute work, -32.6 vs. -49.6; left ventricular stroke work, -50.8 vs. -65.8; left ventricular stroke power, -48.2 vs. -63.1; ballistocardiogram, -34.5 vs. -49.1; aortic dP/dt, -49.7 vs. -65.8; pressure-pulse product, -32.3 vs. -42.3. Heart rate increased less when N2O was included in the mixture: 34.5 vs. 43.6. The lack of response during the addition of N2O to enflurane-O2 is contrary to the significant sympathomimetic response seen when N2O is added to halothane, fluroxene, or diethyl ether. The apparent protection afforded by N2O at equipotent anesthetic levels in small enough that the main consideration in choosing between the 2 mixtures should be the concentration of O2 needed by the patient.