Aging and hyperoxia sensitize heterozygous ND23 mutants to isoflurane toxicity. The percent mortality at 24 h was determined for 30- to 39-day-old wild-type/wild-type (A) and ND23⁶⁰¹¹⁴/wild-type (B) and 21- to 26-day-old ND23^Del/wild-type (C) males and females exposed (+) or not exposed (−) to 2% isoflurane and 75% O₂ for 2 h. Symbols indicate the following: box, second and third quartiles of data; +, mean; horizontal bar, median; and whiskers, minimum and maximum. Significance between control and experimental data was determined using an ordinary one-way ANOVA followed by the Dunnett’s post hoc test for multiple comparisons. *P < 0.05; ****P < 0.0001.