The energy transfer and primary charge separation in photosystem I (PS I) reaction center (RC) particles with an antenna size of 12 chlosophylls/P700 were studied by subpicosecond transient absorption spectroscopy. Upon excitation of the particles at 638 nm under the donor chlorophyll (P700)-neutral conditions, the transition from the excited state of chlorophylls to the charge-separated state of P700(+)A(0)(-) (A(0); acceptor chlorophyll a) proceeded with a time constant of 6.5 ps at similar to 278 K. This time constant was obtained from the absorption increase of P700(+)A(0)(-) at 740 (+/-10) nm, where excited chlorophylls gave only a negligible absorbance change. The distribution of electronic excitation energy among different chlorophyll forms, which was monitored in the wavelength region 650-720 nm, did not show any significant change from 500 fs to 12 ps under both the P700-preoxidized and -neutral conditions. Comparison of the experimental distribution with simulated ones suggests that the excitation energy is distributed within 500 fs highly on P700 and also among other higher-energy chlorophyll forms. On assuming a Boltzmann distribution of excitation energy among all the chlorophyll forms in the particles at 278 K, the intrinsic time constant of the electron transfer from the excited state of P700 to A(0) is estimated to be 3.0 ps.