Enhanced spectral resolution of high-frequency electron paramagnetic resonance (EPR) spectroscopy allows for detailed g-tensor analysis of the active paramagnetic pigments in photosynthetic reaction centers. This analysis has been made for the cation radical and triplet state of the primary donor in photosystem I (P-700) compared to the same species from chlorophyll a (Chla) in vitro. The data prove that, the electronic structures of the primary,donor cation and triplet state are very different from those of monomeric Chla. The g-value anisotropy of P-700(+.) is smaller, and the g, component considerably deviates from that of Chla(+.). The triplet state Of P-700 is also different from Chla(T). In the present study, this difference is resolved because of the high g-value resolution of the high-frequency EPR and characterized by the switching of the directions of the X and Y g-tensor axes with respect to the zero-field axes. These results can be explained either by a delocalized electronic character Of the P-700(+.) and P-700(T) states or by a heteromeric model of the primary donor in photosystem I.