The primary processes in a photosystem I reaction center were studied by fluorescence up-conversion with a subpicosecond time resolution at room temperature. The samples were P700(primary donor chlorophyll)-enriched particles which retained approximate to 14 chlorophylls per P700. Upon selective excitation of P700 at 701 nm at approximate to 5 degrees C, anisotropy of the fluorescence at 749 nm decayed from approximate to 0.3 to approximate to 0.15 with a time constant of 1 ps. The dynamic depolarization is attributed to electronic excitation equilibration between P700 and the surrounding chlorophylls. In the isotropic fluorescence kinetics, at least two decaying components of 2.2 ps (approximate to 35%) and 15 ps (approximate to 55%) were found. The fast and slow components indicate the charge separation before and after full equilibration of excitation energy, respectively. A kinetic model calculation based on the above results suggests that the intrinsic rate constant of the primary electron transfer from P700* is > 0.25 ps(-1).