We show that six mapped recessive lethal point mutations of the Notch locus affect mitochondrial enzyme activities: NADH oxidase, NADH dehydrogenase, succinate dehydrogenase and alpha-glycerophosphate dehydrogenase. The mutant N264-40, which has the same morphological and embryological effects as the Notch8 deletion, demonstrates the same biochemical effects and dosage relations as Notch8. The other five mapped recessive lethals also affect four enzymic activities. They show specific patterns of activity that depend in several cases on the wild-type chromosome in the heterozygous females. That effect occurs with mutants located in the extreme right part of the Notch locus where some mutations, according to other authors, show temperature-sensitive expression.