Horizontal and vertical distributions of nitric oxide (NO) were measured over the Amazon Basin during NASA's Global Tropospheric Experiment (GTE) Amazon Boundary Layer Experiment (ABLE 2A) mission in July--August 1985. During transit flights between the Virginia coast and Manaus, Brazil. NO mixing ratios were 12--15 parts per trillion by volume (pptv) at 5 km altitude. Values up to 200 pptv were observed in electrically active clouds. During longitudinal surveys over the Amazon region, NO mixing ratios in the lower planetary boundary layer decreased from 25--60 pptv over the central basin to 10--12 pptv in coastal regions. In the convective cloud layer or free troposphere, NO mixing ratios averaged 13 pptv in regions not influenced by biomass burning. No longitudinal trend above the mixed layer could be detected. A steep negative gradient with increasing altitude was noted within the mixed layer before midday. Mixing ratios decreased from 60 pptv at 0.2 km to about 15 pptv at the top of the mixed layer (1 km). No further change in mixing ratio with altitude through the convective cloud layer and lower edge of the free troposphere could be detected at this time. By midday, growth of the mixed layer height and enhanced mixing reduced the gradient, and evidence for mixing of NO into the convective cloud layer was noted. ¿ American Geophysical Union 1988