Cimetidine in a dosage of 1000 mg daily (200 mg t.i.d. plus 400 mg nocte) reduced the absorption of protein-bound cobalamin by peptic ulcer patients and normal subjects. However, cimetidine in a dosage of 400 mg at night had no significant effect, nor did tripotassium dicitratobismuthate in a dosage of 480 mg daily. Thus long-term administration of 400 mg cimetidine at night will not produce cobalamin deficiency in man.