The sulfonylureas are a class of oral hypoglycemic agents used to treat type II diabetes mellitus, and tolbutamide is a "first generation" member of this family. It is a nonpolar, weakly acidic drug that binds to serum albumin in the circulation. In the present study, we have examined the interactions of tolbutamide with human serum albumin by isothermal titration calorimetry and heteronuclear multiple-quantum coherence NMR spectroscopy. Calorimetric titrations revealed that tolbutamide binds to albumin at three independent sites with the same or comparable affinity. This result was independently confirmed by NMR experiments which resolved three resonances at 1H chemical shifts of 2.07, 2.11 and 2.14 ppm, corresponding to [methyl-13C]tolbutamide bound to three discrete binding sites. The binding affinity quantitated by calorimetry (Kd = 21 +/- 9 microM at pH 7.4, 37 degrees C) was approximately 5 times lower than the most frequently reported value. Tolbutamide titrations of albumin complexed with three other drugs whose binding sites have been localized by X-ray crystallography (salicylate, clofibric acid, and triiodobenzoic acid) demonstrated direct competition for common binding sites. NMR experiments with samples containing [methyl-13C]tolbutamide and these competing drugs permitted assignment of the resonances at 2.07 and 2.14 ppm to tolbutamide bound to the aspirin sites in albumin subdomains IIIA and IIA, respectively. These findings permit the first assignment of tolbutamide binding sites to specific locations on the albumin molecule within the context of the recently published crystal structure of human serum albumin. In addition, this information provides a molecular basis for predicting unfavorable drug interactions involving tolbutamide in patients with type II diabetes.