The activities of starch synthesizing enzymes were investigated in wheat grains (Triticum aestivumL. cv. Kolibri) throughout the grain development period. Starch phosphorylase (E.C. 2.4.1.1.) activity was especially high during the early period of grain development, while starch synthase I (ADP glucose α‐glucan 4‐α‐glucosyl‐transferase, E.C. 2.4.1.21) had a maximum activity during the later stage of grain filling. The synthetic potential of starch phosphorylase measuredin vitrowas about 16 times higher than the quantity of starch actually produced. It is therefore suggested that starch phosphorylase is of substantial importance in grain starch synthesis, particularly in the early period of grain growth. The synthetic potential of starch synthase I measuredin vitromade up 25 to 50% of the starch production and the synthetic potential of starch synthase II (UDP glucose α‐glucan 4α‐glucosyl‐transferase. E.C. 2.4.1.11) only about 5%.Reducing light intensity (shading) during the grain filling period depressed grain growth and starch production by about 20%. Starch phosphorylase was not significantly affected by the reduced light intensity if enzyme activity is calculated on unit grain weight and not as activity per grain. Starch synthase I activity, however, was depressed by shading during the later stage of grain development. The depressed starch production found under low light conditions, however, cannot only be explained by an affected starch synthase I activity, but probably was also related to other still unknown factors limiting grain growth under low light conditions. The poor starch production in the shaded plants was not due to an insufficient supp