A biogenic hydrocarbon emission inventory system, developed for acid deposition and regional oxidant modeling, is described, and results for a U.S. emission inventory are presented. For deciduous and coniferous forests, scaling relationships are used to account for canopy effects upon solar radiation, temperature, humidity, and wind speed as a function of height through the canopy. Leaf temperature is calculated iteratively from a leaf energy balance as a function of height through the canopy. The predicted light and temperature levels are used with mean empirical emission rate factors and laboratory emission algorithms to predict hydrocarbon emission rates. For application to a U.S. inventory, diurnal emission fluxes of isoprene, alpha-pinene, other monoterpenes, and other hydrocarbons are predicted for eight land cover classes by state climatic division by month. The total U.S. emissions range from 22 to 50 Tg yr/-1 depending upon the formulation of different emission rate factors. Further evaluation of the inventory using field measurements is required to determine the overall accuracy of the emission estimates.