By a highly specific antibody against ochratoxin A (OA), the immunohistochemical fate of po administered OA in Swiss mice given a single dose of 25 mg/kg was examined by light microscopy with two immunoperoxidase methods, an indirect method and a peroxidase-antiperoxidase method. Immunohistochemical staining for OA was demonstrated in the esophagus, stomach, small intestine, kidneys, and liver. OA was most intense in the gastrointestinal tract, intermediate in the kidneys, and least in the liver. In the gastrointestinal tract, OA was demonstrable: (i) in the squamous layer of the esophagus, (ii) in the surface mucus and within macrophages and neutrophils of the stomach lamina propria, (iii) within the cytoplasm of duodenal and jejunal surface epithelial cells as well as within macrophages and neutrophils of the duodenal and jejunal lamina propria. Maximal gastrointestinal tract staining was evident from 5 min to 3 hr, while the esophagus remained stained for 24 hr postdosing. No evidence of OA was obtained in the ileum. In the kidneys, the vast majority of OA was localized within the cytoplasm of the proximal convoluted tubular cells as brown-stained vacuoles, whose size became largest at 3 hr postdosing. Staining for OA was also demonstrated within the epithelium of distal convoluted tubules, the macula densa, the loop of Henle, and sometimes within the epithelium of Bowman's capsule and glomerulus. OA in hepatocytes was most intense between 40 min and 3 hr postdosing. Stained hepatocytes were slightly more concentrated in the periportal area than in the area of the central vein and typically exhibited strong cytoplasmic but weak and infrequent nuclear staining. Biliary excretion of OA was demonstrated, since OA was localized in the lumina of biliary ducts but not within biliary cells.