The electron transfer-induced conformational changes in cytochrome c adsorbed on electrodes have been studied with highresolution surface plasmon resonance spectroscopy. The conformational changes in the native protein follow a stretched exponential function, exp[-(t,tau)(0.5)], suggesting a distribution of the activation barrier heights in the process. The measured time constants, tau, are 0.21 and 0.14 s for the oxidation and reduction, respectively. They are much slower than the electron transfer process, due to structural relaxation of the protein layer. As the concentration of guanidine hydrochloride is increased to 3 M, the conformational changes diminish, which indicates that the electron-transfer center is decoupled from the rest of the protein. After replacing the denaturant with buffer, the conformational changes re-appear but occur with much slower time constants. The slowing-down of the conformational changes may be due to the trapping of the protein in an intermediate state. (C) 2003 Elsevier B.V. All rights reserved.