The fluorescence yield of chlorophyll a of system II in spinach chloroplasts as a function of the fraction q- of reaction centres in the weakly trapping state PQ-, with reduced acceptor Q-, and reduced primary donor chlorophyll, P, of the reaction centre, is described by the function phi = a/(1 - pq-), a and p being constants (Van Gorkom et al. 1978); P was estimated to be 0.74. By special treatment and additions it was ascertained that the donor complex (S-states, see below) was in the reduced state. Three models of pigment systems have been considered: separate units; units with a boundary limiting energy transfer; and the matrix or pigment bed model, which was found to describe the experimental data. The following supplementary assumptions were made: ktf greater than kt greater than k't greater than 0. The rate constant ktf is that for electronic excitation transfer from a chlorophyll a molecule (or reaction-centre chlorophyll) to the surrounding chlorophyll molecules; kt and k't are rate constants for trapping at the reaction centres in the state PQ and PQ-, respectively. From this model and additional data such as fluorescence yield in vivo and in vitro, kt was estimated to be 4 X 10(11) S-1 and k't = 7.1 X 10(10) S-1; ktf greater than 10(12) S-1. In dark-adapted Chlorella, a series of curves respresenting changes in fluorescence yield as a function of time in a succession of six 16 microseconds xenon flashes spaced at 3 s crossed at one point. It is concluded from this and other observations that in the states S2 and S3 (with two or three oxidizing equivalents in the donor complex of system II) a certain fraction of the reaction centres occurs in a special conformational state. In this state electron transfer and, possibly, energy transfer to P+ are appreciably decreased.