The effect of nitrous oxide (N2O) on the MAC of enflurane, halothane, and isoflurane was determined in male rats. Each rat received either enflurane, halothane, or isoflurane, along with 0%, 15%, or 75% N2O. Anesthetic equilibration was verified by mass spectrometry sampling of end-tidal gases. MAC was determined at each N2O concentration by the standard tail clamp method. The N2O dose–response data for each animal were fit by a second-order polynomial equation to estimate the value of a second-order coefficient. A linear dose–response would result in a value of zero, whereas the extent to which the data deviate from nonlinearity would be reflected by an increase in the value of this coefficient. The null hypothesis, that the second-order coefficient should be zero, was tested by a one-sample two-tailedttest. The volatile anesthetic requirement decreased as the N2O concentration increased; however, it did not do so linearly. For each of the three volatile anesthetic groups, the second-order coefficients were consistently greater than zero (P< 0.05). These data are not consistent with the accepted presumption that the summation of N2O with volatile anesthetics is linear.