Manganese chloride (10 mg/kg) was administered orally to guinea pigs for a period of 30 days. The histochemical and histological alterations in gastric mucosa consisted of loss of rnucin and pepsinogen granules, adenosine triphosphatase and glucose‐6‐phosphatase activities and patchy necrosis of the epithelium. The intestinal villi also showed patchy necrosis, the activities of adenosine triphosphatase and glucose‐6‐phosphatase were diminished while there was an increase in the activity of acid phosphatase in the mucosa. Excess of manganese in the gastrointestinal tract produces functional and structural alterations in the mucosal cells. The exact mechanism of action needs further study.