The effect of a reduction of the activity of starch-branching enzyme (1,4-~-D-glucan, 1,4-~-D-glucan-6-glycosyl transferase; EC 2.4.1.18) on photosynthetic starch synthesis and photosynthate partitioning has been studied in leaves of pea (Pisum sativum L.). Leaves of wrinkled-seeded peas, recessive at the rugosus locus (rr), contained lower activity of branching enzyme than leaves of near-isogenic round-seeded peas, dominant at the rugosus locus (RR). Western blots showed that one isoform of the enzyme is absent from rr leaves, corresponding to the isoform that is absent from rr embryos. RR and rr leaves had identical rates of starch synthesis and photosynthesis at low irradiances. At high irradiances the rate of starch synthesis was decreased by up to 40% in rr relative to RR leaves. There was no corresponding increase of sucrose synthesis in rr leaves; instead, the rate of photosynthesis was decreased. This inhibition of photosynthesis was more marked at low than at high temperatures and was accompanied by increased oscillatory behaviour, rr leaves contained higher levels of ADP glucose and glycerate 3-phosphate than RR leaves in low and high light. The contribution of these results to our understanding of the distribution of control in the pathways of starch and sucrose synthesis is discussed.