Ultrastructural studies of the liver and kidneys of rats at 4 and 8 weeks feeding of 4ppm ochratoxin A (OTA) revealed that mitochondria of proximal convoluted tubules (PCTs) in the kidneys of the toxin fed rats were more severely affected, as evidenced by mitochondrial swelling, vacuolation, ring forms, pleomorphism, and disintegration and loss of cristae. The intercellular junctions of PCT epithelial cells were widened and there was complete loss of microvilli. In liver, hepatocytes revealed pyknotic nuclei with increased heterochromatin, degranulation of rough endoplasmic reticulum and vacuolation of cytoplasm besides changes in mitochondria. The changes became more pronounced with increase in duration of toxicity. It may be concluded that OTA causes alternations primarily in mitochondria of renal PCT cells and liver hepatocytes.