A normal population of mice was separated into two groups with reproducibly high (greater than 1.63 atm) or reproducibly low (less than 1.29 atm) nitrous oxide requirements. Males (n = 4) and females (n = 5) with the reproducibly high anesthetic requirements were mated, as were males (n = 4) and females (n = 3) with the reproducibly low anesthetic requirements. The first-generation offspring from parents with the high anesthetic requirements had a higher nitrous oxide ED50 (concentration of nitrous oxide required to abolish the righting reflex in half of the animals) than did offspring from parents with the low anesthetic requirements. Mice with the lowest and the highest anesthetic requirements in the first generation were bred to give the second generation. By repeating this process of breeding, nitrous oxide ED50 testing, and selection of mice with the highest and lowest anesthetic requirements through five generations, the authors were able to breed two groups of mice separated by approximately 0.5 atm in nitrous oxide requirements. This alteration in anesthetic requirement could not be explained by an altered synaptic membrane lipid composition, since no significant difference in synaptic membrane phospholipid, fatty acid, or cholesterol compositions could be detected in the two groups of mice with high and low anesthetic requirements.