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A histological study on small intestine of Wistar rats after oral administration of ricin, a proteinous toxin from castor bean seeds, was carried out. In Experiment I, the jejunum was examined at 1, 2, 5, 10, 15, 20, 24, and 40 hours after oral administration of ricin 30 mg/kg. In Experiment II, ricin was administered at dose of 1, 3, 5, 10, 15, 30, and 60 mg/kg, and after 5 hours the jejunum, mid-portion, and ileum were examined. For comparison, ricin 0.5 mg/kg was administered intraperitoneally and castor bean hemagglutinin (CBH) 30 mg/kg orally. In both experiments, the changes of mucosa were essentially similar dependent on time-lapse and dose respectively, which were atrophy of villus, elongation of crypt, degeneration of epithelium, decrease of goblet cell, fusion of intervillous epithelia, infiltration of neutrophils and eosinophils, and dissociation between epithelium and lamina propria. These changes were most manifest in the jejunum that was contacted with ricin first in a high concentration. In contrast, intraperitoneal administration of ricin caused only dissociation between epithelium and lamina propria and oral administration of CBH caused only milder atrophic changes. The evidences suggest that the mucosal changes by oral administration were caused by direct contact with ricin.