Sorbitol, together with sucrose and starch, is a major final product of photosynthesis in most species of the Rosaceae. However, Little is known concerning the factors affecting the carbon fluxes into these three carbohydrates. The aim of this study was to investigate the effect of net photosynthetic CO2 assimilation rate (NAR) on primary carbon partitioning between sorbitol, sucrose, starch and other metabolites, and on the kinetic pattern of labelling in mature leaves of peach (Prunus persica [L.] Batsch). A pulse-chase labelling experiment was conducted on a group of peach seedlings showing a range of NAR's that were obtained by using bye photosynthetic photon flux density treatments. Mature leaves undergoing one of these treatments were subjected to a 4-min pulse of (CO2)-C-14 followed by a 2- to 120-min chase under (CO2)-C-12. The radioactivity in lipids, proteins and in a residual nonsoluble fraction was negligible. After a 2-min chase the organic ions were strongly labelled, accounting for up to 40% of the total incorporated radioactivity at NAR's below 10.4 mu mol CO2 m(-2)s(-1). After a 30-min chase, most label was found in sorbitol, sucrose and starch with very little radioactivity in the ionic fractions. The analysis of the partitioning of the newly-fixed carbon showed that at low NAR the C-14 flux was mostly into sorbitol rather than into sucrose (ratio = 2.6) or starch. When the NAR was lower than 6.1 mu mol CO2 m(-2) s(-1), starch was lightly labelled. As the NAR increased above 10.4 mu mol CO2 m(-2) s(-1) the (CO2)-C-14 diverted into starch increased dramatically. This study demonstrates that at low NAR's the newly-fixed CO2 is mainly used for sorbitol synthesis. However, as NAR increases, the participation of sucrose and starch in CO2 partitioning are favoured. These results support the hypothesis that primary carbon partitioning depends on photosynthetic rate.