Mixing ratios of nonmethane hydrocarbons, both biogenic and anthropogenic, have been monitored year-round at several sites in the boreal forests of Saskatchewan and Ontario. Seasonal patterns in hydrocarbon mixing ratios are explained by the wintertime effect of the Arctic air mass and the summertime increase in photochemical processing at all sites. Summertime differences in hydrocarbon distributions between sites are indicative of the density of anthropogenic sources and the oxidative capacity of the atmosphere in the region. Consideration of hydrocarbon mixing ratios and OH reaction rate constants suggests that the biogenic hydrocarbon isoprene may limit the oxidative capacity of the atmosphere in boreal regions. The observed temperature dependence of the isoprene mixing ratio suggests that the ambient isoprene concentration is strongly influenced by the emission rate.¿ 1997 American Geophysical Union