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Growth, photosynthesis, utilization of assimilates, and the development of a source function in leaves were studied in relation to changes in concentrations and ratios of phytohormones. Carbon isotope 14C was used to trace utilization and outflow of photosynthetic products from the leaf. Concentrations of endogenous phytohormones were determined by solid-phase immunoenzyme assay. It was shown that, in juvenile leaves (one-fifth of their final area), which did not attain a high rate of photosynthesis, up to 80% of assimilates were incorporated into structural polysaccharides (cellulose and hemicellulose) one day after feeding with 14CO2. During leaf growth and the development of its source function, the synthesis of structural polysaccharides declined to 10%, but the formation of alcohol- and water-soluble compounds (AWSC) grew to 80%. Monosaccharides and oligosaccharides, which could act as transport forms of carbohydrates, constituted 30% and 40% of AWSC, respectively. The percentage of assimilates utilized for protein synthesis decreased with leaf growth. The revealed changes correlate with the concentration and the ratio of free forms of phytohormones at various stages of leaf development. Development of a source function, a decline in cellulose and hemicellulose syntheses, and an increase in AWSC were related to the decrease in ABA and IAA concentrations and the increase in the ABA/IAA ratio. The ABA level can regulate the pathways of photoassimilate utilization in leaves by partitioning carbon flows either to the synthesis of high-molecular-weight compounds (cellulose, hemicellulose, and proteins), used for cell growth in leaves, or to the synthesis of transport forms of carbohydrates. A high ABA level favors the first pathway while low level switches leaf metabolism to the second one.