PD 132301-2, a novel inhibitor of acyl-CoA:cholesterol acyltransferase, is adrenotoxic to several laboratory animal species. Morphogenesis of a zona fasciculata-specific cytotoxicity was evaluated in male Hartley guinea pigs administered 100 mg/kg of PD 132301-2 for up to 7 days. Reversibility of adrenal effects was assessed after a 14-day drug withdrawal period (day 21). Serum cortisol concentrations were determined under basal conditions and after administration of adrenocorticotrophic hormone (ACTH) on days 1, 2, 4, 7, and 21. Isolated foci of cortical cell degeneration and necrosis were apparent in outer zona fasciculata by 2 hr and throughout the zona fasciculata at 6 hr. Early degenerative ultrastructural changes included aggregation of smooth endoplasmic reticulum (SER), variable condensation of mitochondrial matrices and swelling of cristae, partitioning of organelles, autophagosome formation, and disruption of lipid globules. Lesions progressed to locally extensive or diffuse zonal necrosis on days 1 and 2 and near complete ablation of zona fasciculata by day 4. Fasciculata cells remaining on day 4 had reduced numbers and increased size of lipid globules, increased lysosomes, and, occasionally, aggregates of SER and mitochondria. On day 7, SER proliferation and lipid depletion were apparent in remaining cells. ACTH responses were attenuated 24 hr after the first dose, and reduction in basal cortisol levels were seen by 24 hr after the second dose with both effects maximal on day 7. After a 14-day withdrawal period, ACTH responses and adrenal morphology returned to normal. It was concluded that PD 132301-2 induced rapid, reversible, zone-specific, morphologic, and functional adrenocortical effects. Furthermore, mitochondria and SER represented early subcellular targets of toxicity.