Measurements of natural hydrocarbon emission fluxes are reported for a northeastern U.S. deciduous forest, for a northwestern U.S. coniferous forest, and for dominant tree species in the Atlanta, Georgia, region. The emission data were obtained by using a vegetation enclosure procedure and a micrometerologial gradient technique. Isoprene fluxes determined by the gradient method in the deciduous forest varied from 2500 μg/m2/h at 20¿C to 8000 μg/m2/h at 30¿C. Corresponding fluxes obtained with the vegetation enclosure method ranged from 890 to 7300 μm/m2/h. Isoprene fluxes determined by both methods increased exponentially with increasing temperature. Emission rates from isoprene-emitting trees in the Atlanta area were similar to those observed in Pennsylvania. Alpha-pinene fluxes measured in a Douglas fir (Pseudotsuga menziesii) forest ranged from 9 to 1320 μg/m2/h. Relative humidity appeared to directly influence the alpha-pinene emission rate. The reasonable agreement between hydrocarbon fluxes obtained with two independent techniqus is significant evidence of the general validity of current biogenic hydrocarbon emission rates.