The binding of morphine and phenytoin to plasma proteins was examined in healthy subjects and in patients with renal and hepatic failure. In the uremic patients without hepatic failure, morphine binding was dependent on the concentration of total serum proteins and albumin, but not the severity of renal failure as measured by creatinine clearance. Binding of phenytoin, however, was dependent on the degree of renal failure and albumin concentration, but not on total serum protein concentration. Renal transplant in 1 patient restored the binding of both drugs to a value within the normal range. The combination of hypoalbuminemia and hyperbilirubinemia resulted in the greatest impairment to binding for both drugs. It is concluded that patients with uremia, jaundice, hypoalbuminemia, particularly in combination, are sensitive to usual clinical doses of morphine, at least in part, because of decreased binding to plasma proteins.