In the understory of a tropical rainforest, light flecks can contribute 10-80% of the total light flux. We investigated the capacity of eight shade-tolerant species to use light flecks by examining the time required for full induction of photosynthesis during an artificial light fleck. CO2 fixation rates were measured with a portable LiCor gas-exchange system for plants growing in the field on Barro Colorado Island, Panama. In all species induction to 50% of maximum CO2 fixation occurred quickly, from 1 to 3 min. In species with short leaf lifetimes (1 year), induction to 90% of maximum also occurred quickly, in 3-6 min. In contrast, the species with longer lived leaves (>4 years) required 11-36 min for induction to 90% of maximum. Induction times for leaves from gap and understory plants of the same species were indistinguishable. Elevated CO2 did not eliminate the slow induction phase of long-lived leaves. This suggests that slow induction did not result from stomatal limitation. O2 evolution, measured on excised leaf disks, induced in 3-4 min in species with short-lived leaves, and 4-8 min in species with long-lived leaves. The rapid induction of O2 evolution indicates that the slower induction of CO2 fixation in long-lived leaves was not caused by a delay in the induction of electron transport. Activation of rubisco may be the major factor limiting response times in species with long-lived leaves. Species from Panama with short-lived leaves had remarkably rapid induction times that are comparable to those of algae or higher plant chloroplasts. We also found that understory forest plants induced two to seven times more quickly than did potted plants.