Chemiosmotic coupling has been widely accepted as the basis of understanding of the mechanism of photophosphorylation. One of the most significant evidence was contributed by Jagendorf and Uribe [ 11, who demonstrated acid-base induced synthesis of ATP from added ADP and Pi in the absence of light- dependent electron transport. However, the mechanism of ATPase reaction involved in the final step of energy transformation, seems to be unsatisfactorily solved by the chemiosmotic hypothesis. Chloroplast coupling ATPase (CF,) was shown to contain tightly bound adenine nucleotides which participate in photophosphorylation [2-41. Recently in analogy to oxidative phosphorylation, a con- formational hypothesis was put forward by Harris and Slater [4]. In their model the energy requiring process of the phosphorylation cycle is believed to be an electron transport-induced conformational change of CF, which leads to release of tightly bound ATP. By assuming certain equilibria between free and CFI - bound adenine nucleotides and P,, the authors pre- dicted that tightly bound ATP may be formed from ADP and Pi without further energy requirement. In the present paper energy-dependent release of adenylates tightly bound to CF, is demonstrated. Energy can either be supplied by light-dependent photosynthetic electron transport or by an acid-base transition in the dark. The results indicate that chemiosmotic and conformational hypotheses may not be regarded as alternative [5] but additive con- cepts for the comprehension of the mechanism of photophosphorylation. 194 2.