The onset of photophosphorylation at the various stages of greening showed different patterns with varying concentrations of Pi. With further greening, ATP formation occurred at the lower concentration of Pi (48 hrs; 0.05 mM). At early stages of greening, more Pi was required for photophosphorylation (6 hrs; 5.0 mM). The addition of cell-free extracts of etiolated barley seedlings resulted in the competitive inhibition of photophophorylation. The apparent inhibition by cell-free extracts were gradually decreased during greening of etiolated barley seedlings. We found that the inhibitors of photophosphorylation in cell-free extracts were some organic phosphates and most of them was P-choline. P-choline inhibited photophosphorylation competitively with Pi and its content was decreased considerably in greening. It is likely that P-choline partly delay the photophorylation in early stages of greening.