Mitochondrial toxicity was assessed in the brains of developingErythrocebus patasmonkey fetuses exposed in utero to the nucleoside analogue drug zidovudine (3´-azido-3´deoxythymidine or AZT). PregnantE. patasmonkeys were given 0 (n= 5), 10 (n= 3), and 40 (n= 3) mg of AZT/day, equivalent to 21 and 86% of the human daily dose, for the last half (about 10 weeks) of gestation. Mitochondria were isolated from fetal cerebrum and cerebellum at birth and mitochondrial morphology was examined in these tissues by transmission electron microscopy (TEM). Oxidative phosphorylation (OXPHOS) enzyme specific activities were measured spectrophotometrically. Mitochondrial DNA (mtDNA) integrity and quantity were determined by Southern blot and slot blot analysis. In the cerebral mitochondria, reduced nicotinamide adenine dinucleotide (NADH) dehydrogenase (complex I) specific activity decreased by 25% in monkeys treated with 40 mg of AZT/day compared with unexposed monkeys (p≥ .05). At the same AZT dose in the cerebral mitochondria, succinate dehydrogenase (complex II) and cytochromecreductase (complex IV)–specific activities showed dose-dependent increases (p≥ .05), compared with those in controls. In the cerebellum, no difference was seen in mitochondrial OXPHOS enzyme activities between unexposed and exposed fetuses. Furthermore, TEM demonstrated no difference in mitochondrial morphology in frontal cerebrum or cerebellum from unexposed and exposed fetuses, and all fetuses had similar amounts of mtDNA in both tissues. Cerebral mtDNA degradation was noted in the highest AZT dosage group, whereas mtDNA from cerebellum was uneffected. Thus, in fetal patas monkeys given a human equivalent daily dose of AZT during the last half of pregnancy, mitochondria in the fetal cerebrum appear to sustain moderate damage, while the fetal cerebellum mitochondria were not effected.