The radical cations of [15N]bacteriochlorophyll a and of the primary donor P-865 in N-15-labeled reaction centers of Rhodobacter sphaeroides R-26.1 were investigated in frozen solutions using two-dimensional ESEEM methods. Both three-pulse (stimulated echo) and four-pulse (HYSCORE) sequences were employed to avoid ambiguities in data analysis. Computer simulations of the experimental powder spectra were performed, yielding a complete set of nitrogen hyperfine coupling tensors for both systems. The obtained tenser values are compared with those from ENDOR measurements and from semiempirical INDO-type MO calculations. The results obtained from 2D stimulated echo ESEEM and HYSCORE for P-865(.+) are interpreted in terms of an asymmetric spin density distribution over the halves of the bacteriochlorophyll dimer ("special pair") in the reaction center with a ratio of approximately 5:1. This asymmetry is considerably larger at low temperatures in the frozen state than at room temperature. It is postulated that two different conformational states of the dimer exist at these temperatures with different spin density distributions.