The MAC of halothane, isoflurane, and enflurane was determined using the tail-clamp technique in pregnant female, nonpregnant female, and male Swiss Webster mice (n = 216) and Sprague–Dawley rats (n = 112). Mean MAC values (±SD) for halothane, isoflurane, and enflurane in mice were 0.95 · 0.07%, 1.34 · 0.10%, and 1.95 · 0.16%, respectively; values in rats were 1.03 · 0.04%, 1.46 · 0.06%, and 2.21 · 0.08%, respectively, all significantly higher than in mice. Neither the sex of the animals nor whether female animals were pregnant influenced the results.