Treatment of male Sprague-Dawley rats with N-nitrosomorpholine (NNM) in the drinking water at a dose of 120 mg/l for 7 weeks resulted in a subsequent enhanced development of focal and nodular lesions in the adrenal cortex. Sequential observation revealed that focal lesions in the zona reticularis/fasciculata or the zona glomerulosa developed both earlier and at a significantly higher incidence in animals treated with carcinogen than in untreated controls. Foci observed within or adjacent to the zona glomerulosa were all of pale cell appearance and contained large numbers of electron-dense cytoplasmic granules similar to those observed in normal granulosa cells. The foci and nodules which arose in the zona reticularis/fasciculata were, in contrast, characterized by a reduction or loss of the dense osmiophilic droplets normally seen in the cells of this region of the adrenal cortex, a pronounced increase in pleomorphic mitochondria of atypical appearance and the development of vacuoles.