Gas exchange responses to rapid changes in light were studied in a C(3) tree, Claoxylon sandwicense Muell-Arg and a C(4) tree, Euphorbia forbesii Sherff that are native to the understory of a mesic Hawaiian forest. When light was increased to 500 micromoles per meter per second following a 2 hour preexposure at 22 micromoles per meter per second, net CO(2) uptake rates and stomatal conductance gradually increased for over 1 hour in C. sandwicense but reached maximum values within 30 minutes in E. forbesii. Calculation of the intercellular CO(2) pressures indicated that the primary limitation to CO(2) uptake during this induction was nonstomatal in both species. The photosynthetic response to simulated sunflecks (lightflecks) was strongly dependent on the induction state of the leaf. Total CO(2) uptake during a lightfleck was greater and the response was faster after exposure of the leaf to high light than when the leaf had been exposed only to low light for the previous 2 hours. During a series of lightflecks, induction resulted in increased CO(2) uptake in successive lightflecks. Significant postillumination CO(2) fixation was evident and contributed substantially to the total carbon gain, especially for lightflecks of 5 to 20 seconds' duration.