The serum protein binding properties of prazosin [a drug used widely in the chronic treatment of hypertension] were determined by the equilibrium dialysis technique, using radiolabeled drug of high specific activity. At concentrations typically used in human therapy, prazosin bound to serum proteins to .apprx. 92%. This value was not altered in the presence of other drugs [chlordiazepoxide, chlorpropamide, dicumarol, methyl dopa, phenobarbital, phenytoin, polythiazide propranolol] frequently diminished in conjunction with prazosin.