Out-of-phase electron spin echo and free induction decay after selective excitation were studied for a spin-correlated radical pair P(700)(+)A(1)(-) in the PSI photosynthetic reaction center. Spin relaxation between the pair sublevels at the initial delays after laser flash was found to be much faster than at the longer delays. This initial fast relaxation in combination with chemical decay of the pair results in inversion of sublevel populations. Temperature dependence of this relaxation is much weaker than that for 'normal' spin-lattice relaxation. (C) 2002 Elsevier Science B.V. All rights reserved.