The relationship between photointermeidates of bacteriorhbdopsin (bR) and proton uptake was investigated by a flash photolysis technique. Attention was paid to keep the sample in a light-adapted state and under the same conditions during measurements. The decay of M-intermediate (M), the formation and decay of O-intermediates (O), the recovery of bR and release and the uptake of protons were measured with a purple membrane suspension (pH 7.0, 0.5 M NaCl) at various temperatures. The following reaction scheme for the bR photocycle was deduced from the results. 1) The decay process of M consists of three components (M_, M_ and M_s). 2) O is formed from the faster component of M (M_). 3) A part of M (M_ and M_s) decays directly to bR without passing through O. 4) The uptake of protons is coupled with the decay of the slowest M (M_s). The above reaction scheme was confirmed by the results obtained from similar experiments using an analog pigment of bR, Np-bR, M of which decays much more slowly than that in native bR system
