A general NMR method is presented that allows a precise determination of the second-order rate constant, k(ese), for the electron self-exchange in blue copper proteins, from the longitudinal relaxation rates of the nuclei in the protein. The method relies on the use of partly oxidized (paramagnetic) samples of the protein, In contrast to previous NMR approaches for the determination of electron self-exchange rates, the applicability of the method extends beyond the slow-exchange limit, k(ese)c much less than R-ip, i = 1, 2, where c is the protein concentration, and R-ip is the paramagnetic relaxation enhancement of the observed nuclei.