1. Tissue accumulation of the antihyperglycaemic agent metformin (dimethylbiguanide) was examined after oral administration to the normal and streptozotocin (STZ) diabetic mouse. 2. Metformin (50 mg/kg body weight containing 14C-metformin 25 microCi/kg body weight), which is stable and not metabolized, resulted in maximum plasma concentrations at 0.5 h which declined to 1000 mumol/kg wet weight at 0.5-2 h, but declined to < 2% of maximum by 24 h. 4. Stomach, colon, salivary gland, kidney and liver accumulated metformin more than two-fold, and concentrations of the drug in heart and skeletal (gastrocnemius) muscle were greater than plasma concentrations on some occasions up to 8 h. 5. In a separate study, i.v.-administered metformin was selectively accumulated by tissues of the small intestine. Thus, retention of metformin by tissues of the small intestine may represent a deep compartment for the drug.