Hydrophobic interactions play an important role in binding S-(N-aryl/alkyl-N-hydroxycarbamoyl)glutathiones to the active sites of human, yeast, and Pseudomonas putida glyoxalase I, as the log K(i) values for these mechanism-based competitive inhibitors decrease linearly with increasing values of the hydrophobicity constants (pi) of the N-aryl/alkyl substituents. Hydrophobic interactions also help to optimize polar interactions between the enzyme and the glutathione derivatives, given that the K(i) value for S-(N-hydroxycarbamoyl)glutathione (pi = 0) with the human enzyme is 35-fold larger than the interpolated value for this compound obtained from the log K(i) versus pi plot. Computational studies, in combination with published X-ray crystallographic measurements, indicate that human glyoxalase I binds the syn-conformer of S-(N-aryl-N-hydroxycarbamoyl)glutathiones in which the N-aryl substituents are in their lowest-energy conformations. These studies provide both an experimental and a conceptual framework for developing better inhibitors of this antitumor target enzyme.