The electron spin-lattice relaxation behavior of the oxidized bacteriochlorophyll a dimer in reaction centers from Rhodobacter sphaeroides has been examined by the method of saturation-recovery EPR over the temperature range 3.8 K less-than-or-equal-to T less-than-or-equal-to 22 K. Its spin-lattice relaxation is nonexponential due to an orientation-dependent dipolar interaction with the non-heme Fe(II) of the reaction center. The saturation-recovery EPR traces were fit by using an equation which models the recovery in terms of a sum of isotropic (scalar) and orientation-dependent (dipolar) rate constants. The center-to-center distance between the bacteriochlorophyll a dimer and the non-heme Fe(II) is 28 angstrom and it is found that the Heisenberg exchange interaction is too small to make a measurable contribution to the scalar relaxation rate of the oxidized bacteriochlorophyll a dimer. The, scalar relaxation rates for the oxidized bacteriochlorophyll a dimer show a T1 temperature dependence which differs significantly from that of model porphyrin radicals. It appears that the unusually rigid protein environment surrounding the bacteriochlorophyll a dimer produces a strong coupling between the spin transitions of the radical and the low-frequency vibrational modes of the lattice. The dipolar mte constants of the oxidized bacteriochlorophyll a dimer and those of the stable tyrosine radical, Y(D)., in Mn-depleted photosystem II show the same temperature dependence. This confirms the assignment of the non-heme Fe(II) as the source of relaxation enhancement for Y(D). in Mn-depleted photosystem II and shows that the spin relaxation properties of the non-heme Fe(II) species in the two proteins are very similar. Using the relative magnitudes of the dipolar rate constants in the two proteins and the distance between the bacteriochlorophyll a dimer and the non-heme Fe(II) in the bacterial reaction center, we calculate a Y(D).-Fe(II) distance of 37 +/- 5 angstrom in photosystem II. This agrees well with the distance predicted from the structure of the bacterial reaction center.