In June 1993, net photosynthetic rates, stomatal conductance and isoprene emission rates of sweetgum leaves (Liquidambar styraciflua L.) were measured at the top of the forest canopy (sun leaves) and within the canopy at a height of 8-10 m above ground level (shade leaves). Large differences in net photosynthetic rates and stomatal conductance were found between sun and shade leaves. Mean rates of isoprene emission, expressed on a leaf area basis, were significantly lower in shade leaves than in sun leaves (4.1 versus 17.1 nmol m(-2) s(-1)); however, because specific leaf area of sun leaves was lower than that of shade leaves (0.0121 versus 0.0334 m(2) g(-1)), the difference between sun and shade leaves was less, though still significant, when isoprene emissions were expressed on a dry mass basis (45.5 versus 29.0 micro g C g(-1) h(-1)). Saturation of both net photosynthesis and isoprene emission occurred at lower PPFDs in shade leaves than in sun leaves. The effect of leaf temperature on isoprene emissions also differed between sun and shade leaves. Sun leaves lost a significantly greater percentage of fixed carbon as isoprene than shade leaves. The leaf-level physiological measurements were used to derive parameters for a canopy-level isoprene flux model. The importance of incorporating differences between sun- and shade-leaf properties into existing models is discussed.