Recent determinations of the minimum alveolar anesthetic concentration (MAC) for nitrous oxide in rats yield values of 1.51-1.55 atm. When combined with results from other reports, these results suggest a deviation from linear additivity, and call into question the unitary theory of narcosis. The present report provides evidence that nitrous oxide does act in an additive manner. We directly determined the MAC for nitrous oxide in groups of 10 Sprague-Dawley and Long-Evans rats, using electrical stimulation of the tail or abdomen. MAC equaled 2.35 +/- 0.20 atm (mean +/- SD) in Long-Evans rats with tail stimulation; 2.21 +/- 0.19 in Sprague-Dawley rats using tail stimulation; and 1.99 +/- 0.21 atm in Sprague-Dawley rats using abdominal stimulation (tail versus abdominal stimulation was significantly different). Our MAC values are higher than those recently reported by others. Differences from the previous reports may be explained by differences in experimental design or interpretation. Our findings are consistent with an additive effect of nitrous oxide with other inhaled anesthetics (data from other reports) and thereby support the unitary theory of narcosis.