Summary Sucrose-P synthetase was extracted and assayed in crude preparations from leaves of various species that differ in leaf starch accumulation. Magnesium was required in the grinding medium for enzyme stability and in the assay medium for enzymic activity. The enzyme, extracted from leaves of tobacco (Nicotiana tabacum L.), peas (Pisum sativum L.), beans (Phaseolus vulgaris L.) and peanuts (Arachis hypogaea L.) had relatively low activity on a chlorophyll basis (6.8 to 12.9,umol product/mg chlorophyll h), and its activity was inhibited by 50 mM sucrose (30 to 100 Ofo inhibition). In contrast, sucrose-P synthetase activity in leaf extracts of wheat (Tritium aestivum L.), barley (Hordeum vulgare L.) and spinach (Spinacea oleracea L.) was generally higher (10 to 12.5,µmol/mg chlorophyll h) and its activity was not inhibited by sucrose. Activity of sucrose synthase in leaf extracts of the different species was variable (0 to 3.4 ,umol!mg Chl h), and generally much lower than the activity of sucrose-P synthetase. Leaf starch content of the different species was negatively correlated (r = -0.94) with the activity of sucrose-P synthetase activity in leaf extracts. The results suggested that the activity of sucrose-P synthetase may be ratelimiting for photosynthetic sucrose formation and, hence, may account for species differences in leaf starch accumulation. Sucrose inhibition of this enzyme in some species may also provide a biochemical explanation for increased leaf starch accumulation when translocation is reduced