Diel (24 h) courses of net CO2 exchange of leaves were determined in eight species of tropical rainforest plants on Barro Colorado Island Panama during 1990 and 1991. The species included three canopy trees one liana two epiphytes and one hemiepiphyte. One of the species studied was growing in a rain-forest gap. Daily carbon gain varied considerably across species leaf age and season. The analysis of data for all plants from 64 complete day/night cycles revealed a linear relationship between the diurnal carbon gain and the maximum rate of net CO2 uptake Amax. Nocturnal net carbon loss was about 10% of diurnal carbon gain and was positively related to Amax. We conclude that short-term measurements of light-saturated photosynthesis performed at periodic intervals throughout the season allow the annual leaf carbon balance in these rain-forest plants to be predicted.