The delta C-13 and deltaD values of newly emerging to senescing tree leaves produced during a rainy season were obtained in dry seasonal and moist forest in Panama. Newly emerging leaves had less negative delta C-13 values than older leaves yet instantaneous p(i)/p(a) was never lowest in the youngest leaves. Furthermore, isotopic enrichment during early growth may have a detectable influence on the delta C-13 values of mature leaves. The deltaD values ofcellulose nitrate were only related to deltaD values of leaf water if leaf age was also considered so that, for a given deltaD of leaf water, deltaD values were highest in the youngest leaves (R-2 = 98%). There was no correlation between leaf age and deltaD values of leaf water. Investment of translocated organic carbon is a factor likely to be associated with both C-13 and deuterium enrichment effects in new leaves. A coarse, mass balance approach can estimate the proportional heterotrophic investment in leaf growth and improve estimates of integrated p(i)/p(a) by approximating delta C-13 for the most autotrophic phase of leaf growth. delta C-13 values of the predominantly sucrose mobile organic fraction in new leaves were less negative than in older leaves, thereby suggesting that the enrichment did not occur at the original site of production of the substrate for new leaf growth. Although the delta values of early leaf growth must be influenced by inputs of translocated organic carbon, enrichment effects, per se, are apparently caused by other mechanisms such as, for carbon, de novo sucrose synthesis and anaplerotic replenishment. Better recognition of metabolic causes of isotopic enrichment in leaves promises to increase the power and accuracy of inferences about carbon and water use of tropical trees from delta analyses.