Increases in the chlorophyll fluorescence F(o) (dark level fluorescence) during heat treatments were studied in various higher plants. Besides the dissociation of light-harvesting chlorophyll a/b protein complexes from the reaction center complex of PS II and inactivation of PS II, dark reduction of Q(A) via plastoquinone (PQ) seemed to be related to the F(o) increase at high temperatures. In potato leaves or green tobacco cultured cells, a part of the F(o) increase was quenched by light, reflecting light-induced oxidation of Q(A) (-) which had been reduced in the dark at high temperatures. Appearance of the F(o) increase due to Q(A) reduction depended on the plant species, and the mechanisms for this are proposed. The reductants seemed to be already present and formed by very brief illumination of the leaves at high temperatures. A ndhB-less mutant of tobacco showed that complex I type NAD(P)H dehydrogenase is not involved in the heat-induced reduction of Q(A). Quite strong inhibition of the Q(A) reduction by diphenyleneiodonium suggests that a flavoenzyme is one of the electron mediator to PQ from the reductant in the stroma. Reversibility of the heat-induced Q(A) reduction suggests that an enzyme(s) involved is activated at high temperatures and mostly returns to an inactive form at room temperature (25 degrees C).