We used gas exchange techniques to estimate maximum rate of carboxylation (Vcmax), a measure of photosynthetic capacity, in the understory and upper crown of a closed deciduous forest over two seasons. There was extensive variability in photosynthetic capacity as a result of vertical canopy position, species type, leaf age and drought. Photosynthetic capacity was greater in oaks than in maples and greater in the overstory than in the understory. Parameter Vcmax was maximal early in the season but declined slowly throughout most of the summer, and then more rapidly during senescence. There was also an apparent decline during drought in some trees. Variability in Vcmax as a result of species or vertical canopy gradients was described well by changes in leaf nitrogen per unit area (Na). However, temporal changes in Vcmax were often poorly correlated with leaf nitrogen, especially in spring and summer and during drought. This poor correlation may be the result of a seasonally dependent fractional allocation of leaf nitrogen to Rubisco; however, we could not discount Rubisco inactivation, patchy stomatal closure or changes in mesophyll resistance. Consequently, when a single annual regression equation of Vcmax versus Na was used for this site, there were substantial errors in the temporal patterns in Vcmax that will inevitably result in modeling errors.