The development of photosystem I activity of plastids isolated from greening barley (Hordeum distichum, L.) leaves was studied. The electron transport activity in photosystem I was measured as anthraquinone-mediated oxygen uptake and as light induced absorbance changes of the reaction centre molecule P700. P700 oxidation was observed after one hour of greening though an electron transport leading to oxygen uptake was observed after 30 minutes. Phenazine methosulphate had no effect on the oxidation of P700 until after four hours of greening. The ratio chlorophyll/P700 decreased from about 2300/l at one hour to 640/l at sixteen hours of greening. The light intensity dependence of the electron transport of photosystem I showed that the photosynthetic units gradually increased in size as the greening proceeded. The increase of the rate of the oxygen uptake, calculated on plastid basis, decreased after eight hours while the P700 content, calculated on plastid basis, increased continuously between three and sixteen hours. Chromatographic separations and fluorimetric analyses of the chlorophyll pigments showed that the reaction centre molecule could not be protochlorophyllide or chlorophyllide.