During acrylamide and hexacarbon exposure few changes have been reported in motoneuron perikarya. In the present study, light microscopic examination of lumbar motoneurons from rats intoxicated with either 2,5-hexanedione (2,5-HD) or acrylamide showed relatively few nonspecific changes compared to controls. However, ultrastructural study of 2,5-HD-intoxicated perikarya revealed a range of cytological reorganization: nuclear eccentricity and capping, reduced numbers of large Nissl bodies, and mitochondrial hypertrophy and hyperplasia. In 2,5-HD-intoxicated perikarya, computer-assisted stereologic analysis demonstrated a significant increase in the volume density of mitochondria. Ultrastructurally, acrylamide-intoxicated perikarya showed a marked reduction in the size of Nissl bodies. Stereologic analysis showed reductions in Nissl bodies, granular endoplasmic reticulum and Golgi complexes, and an increase in mitochondria. Taken together, these qualitative and morphometric changes, which were not obvious on light microscopic examination, imply significant reorganization of perikaryal metabolism.