Rapid photochemical electron injection into unfolded ferricytochrome c titrated with 2.3 to 4.6 M guanidine hydrochloride (GuHCl) at pH 7 and 40 degrees C produced unfolded ferrocytochrome, which then converted to the folded protein. Two folding phases were observed : a fast process with a time constant of 40 microseconds (4.6 M GuHCl), and a slower phase with a rate constant of 90 +/- 20 per second (2.3 M GuHCl). The activation free energy for the slow step varied linearly with GuHCl concentration; the rate constant, extrapolated to aqueous solution, was 7600 per second, Electron-transfer methods can bridge the nanosecond to millisecond measurement time gap for protein folding.