Photosystem I particles from spinach were studied with linear-dichroic absorbance-detected magnetic resonance in zero-magnetic field. The microwave-induced triplet-minus-singlet (T-S) spectra and the linear-dichroic (LD) T-S spectra were recorded for the D + E and the D - E microwave transitions of the triplet state of the primary donor, P-3(700). From these data the directions of the optical transition moments contributing to the T-S spectra in the 600-750 nm region, with respect to the triplet x- and y-axes of P-3(700) were obtained. The orientation of the optical Q(Y)-transition moment of P-700 relative to the triplet x- and y-axis is found to be 35 +/- 2 degrees and 56 +/- 1 degrees, respectively. A comparison is made with data obtained for monomeric chlorophyll (Chl) a in two glasses. The orientation of the Q(Y)-transition moment with respect to the in-plane triplet x- and y-axes of P-3(700) differs from monomeric Chl a in the two glasses. This difference is ascribed to the different environments of P-700 and Chl a, rather than to the dimeric structure of P-700. In addition to the Q(Y)-absorption band of P-700, the T-S and LD(T-S) spectra contain features that are ascribed to transitions involving accessory Chls. The contribution of the Q(Y) transition of the primary acceptor to a band at 687 nm in the T-S spectrum is discussed, and a comparison is made with the signal of the primary acceptor in the T-S spectrum of Heliobacterium chlorum.