Acute toxicity of bis (tributyltin) oxide in the sweat glands in the rat footpad was investigated by electron microscopy and an energy-dispersive X-ray microanalyzer. Male Wistar rats received an intramuscular injection of 0.5 ml/kg bis (tributyltin) oxide. After 6-8 h, swelling of mitochondria appeared in the secretory cells of the sweat glands. After 12 h, the secretory cells began to show intracytoplasmic edema. After 16-20 h, secretory cells in some sweat glands showed marked hydropic degeneration with swollen cytoplasm. Using X-ray microanalysis, tin peaks were preferentially obtained from the swollen mitochondria of the affected secretory cells. Mitochondria dysfunction due to the toxic effects of bis (tributyltin) oxide induced changes in the secretory cells of rat sweat glands. After 24-48 h, the secretory portion of the sweat glands contained three types of cells: degenerating dark cells, regenerating cells carrying injured mitochondria, and light cells which were morphologically very similar to the cells in the transitional portion of the sweat gland. These light cells appeared to differentiate into active secretory cells after settling down in the secretory portion. Based on these observations, we concluded that the cells in the transitional portion could play an important role at least as reserve cells against secretory cell toxicity. In association with the regenerating process of the damaged secretory portions, increased mitotic activities were seen in different areas of all the dermal sweat ducts. The above-mentioned morphological observations for cell damage and subsequent regeneration and renewal ofsecretory cells in sweat gland intoxication have not been reported so far.