Measurements of carbon monoxide (CO) and methane (CH4) were made in the North American Arctic during July--August 1988. The distribution of CH4 was variable in the atmospheric mixed layer (0--2 km), with concentrations determined primarily by interactions of biogenic emissions from wet tundra and turbulent mixing processes. Carbon monoxide exhibited little variation in unpolluted mixed layer environments indicating a minor role for biogenic sources and/or sinks in determining its distribution. In the free troposphere (2--6 km) both CO and CH4 were variable. Concentration gradients were most frequently associated with intrusions of upper tropospheric or stratospheric air into the midtroposphere, emissions from forest and tundra fires, and long-range transport of enhanced concentrations of these gases from unidentified sources. Summertime haze layers exhibited midtropospheric enhancements of CH4 similar to those measured in winter Arctic haze events. However, these summer pollution episodes did not exhibit positive correlations with particulate sulfate. The summer Arctic and sub-Arctic haze events observed during the Arctic Boundary Layer Expedition (ABLE 3) were primarily a result of forest and tundra fires of natural origin. The tendency for relatively high variability of CO and CH4 at altitudes of 3--6 km indicates that ground-based monitoring will not provide an adequate assessment of the chemical composition of the Arctic troposphere to support future global change studies. ¿ American Geophysical Union 1992