The effects of a 0.5 g/kg body weight arginine infusion on plasma inorganic phosphates and potassium were examined in nineteen normal subjects. Plasma phosphorus displayed a highly significant (p less than 0.001) fall with a maximum depression below baseline of 1.11 +/- 0.15 mg/100 ml or 33 +/- 3% (mean +/- SEM); there was a significant correlation (p less than 0.01) between this fall and the insulin peaks induced by arginine. Plasma potassium levels displayed a distinct and significant increase in eleven of the twelve subjects studied; the maximum increase above baseline was 1.02 +/- 0.14 mEq/1 or 27 +/- 4.5% (p less than 0.001). No change occurred in blood pH values determined in four subjects. In six normal subjects, the test was repeated with the addition of somatostatin (250 micrograms bolus, followed by 500 micrograms/hr), which abolished the insulin and growth hormone response to arginine. It also abolished the fall in plasma phosphorus but appeared (if anything) to augment the increase in potassium. These findings show that arginine is responsible for a fall in plasma phosphorus related to the insulin response, and for an increase in plasma potassium of clinical significance, the mechanism(s) of which, however, are still obscure.