Resumo. Light availability is one of the main factors that limits photosynthetic rates in saplings in the forest understory. The aims of this study were to determine how photosynthetic parameters (maximum carboxylation velocity of Rubisco – V c-max and maximum rate of electron transport – J max ) respond to irradiance, temperature and diurnal variations in the physical environment. Gas exchange data were collected between 08h00 and 16h00, and from September to November (2010) under ambient conditions, temperature of 27± 2 ºC and relative humidity of 70 ± 3%. CO 2 saturated photosynthesis (A pot ) was measured under light saturation and [CO 2 ] of 2000 μ mol mol -1 . Data were subjected to analysis of variance and then the Tukey test was used for mean separations. V c-max and J max correlated with photosynthetic rates at [CO 2 ] of 380 μ mol mol -1 (A 380 ) and A pot . The increase in intercellular CO 2 concentration (C i ) led to increase in photosynthetic rates, up to the point of photosynthesis limitation by the electron transport rate. Below the collimation point (300-569 μmol mol -1) photosynthetic rates were limited by V c-max . Increases in irradiance and temperature in the forest understory led to increasing the values of A 380 , A pot , V c-max and J max . However, the effect of time of day on V c-max and J max was not significant ( p > 0.05). Absence of an effect of time of day on V c-max and J max is important for the leaf to efficiently use the sun flecks that occasionally puncture the forest floor.