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Measurements of carbon monoxide (CO) were made over the Amazon Basin of Brazil during the 1987 wet season as part of the NASA Global Tropospheric Experiment/Amazon Boundary Layer Experiment (ABLE 2B). The distribution of CO over the altitude range sampled (0.15–4.5 km) was influenced by surface emissions from biological sources, long-range transport of pollutants from northern hemisphere sources, and by transport processes associated with local convective mixing. Surface sources are indicated by a qualitative interpretation of the typical pattern of decreasing concentrations with increasing altitude and increasing concentrations of CO at 0.15-km altitude during a transect from the Atlantic coast to the central basin. Atmospheric convective activity produced irregular patterns of variability at time scales of less than 1 hour over a localized area. The disruption of mixed layer growth and decay processes has a particularly important influence on CO concentrations in the daytime lower troposphere. Intrusions of northern hemisphere air into the central Amazon resulted in increased CO concentrations in the lower and midtroposphere. The correlation of CO with O3 was positive under conditions interpreted as being influenced by northern hemisphere air, and negative during all other meterological conditions experienced in ABLE 2B.