The effect of cytokinins benzylaminopurine (BAP) and thidiazuron on photosynthetic rate and enzyme activities in sugar beet, pea, meadow fescue and reed fescue leaves was studied. Low concentrations (3-10 g nr3) stimulated the net photosynthetic rate, the activities of ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBPCO) and other enzymes of the carboxylation phase of Calvin cycle, i.e. ribose phosphate isomerase and phosphoribulokinase, and the cycle including the NADP-glyceraldehyde phosphate dehydrogenase complex. On the contrary, there was no change in phosphoenolpyruvate carboxylase activity. In treated leaves the quantity of RuBPC increased along with increases in the level of total leaf proteins. Thus cytokinins act, probably, by specific altering of the synthesis of certain proteins. Inducing or stimulating the synthesis of essential proteins involved in photosynthetic processes, cytokinins cause changes in photosynthetic activity.