Exchange of NO and NO2 was studied in two tropical ecosystems in Venezuela. Fluxes were obtained from temporal changes of the concentrations in a closed chamber placed over the soil surface. In the savanna the net emission of NO ranged from 3 to 15 ng N m-2 s-1 the median value being about 8 ng N m-2 s-1. There was a net emission in terms of NOx (NO+NO2). The surface resistance to uptake of NO2 on the ground was around 0.2 s mm-1. With the average concentrations prevailing at the site the NO2 deposition was estimated to be less than 50% of the median NO emission. The NO emission showed a weak diurnal variation. The soil was very dry and when the soil was artificially watered (to simulate between 3 and 15 nm of rainfall) the emission of NO increased dramatically to 10 to 20 times the background values. It stayed high when water was supplied daily for 10 days. Addition of nitrate to the soil resulted in a 100- to 300-fold increase of the NO emission. Burning also resulted in a large increase in the emission of NO from the soil. It stayed high for several days after the burning. These results indicate that tropical savannas may constitute a very important part of the natural source of atmospheric NOx on regional or even global scale. The estimated annual emmission from the savanna were 0.4 to 0.5 g N m-2. The sum of the wet and dry deposition was estimated to be less than 0.2 mg N m-2 indicating a net export of NOx-N from this region. The NO emission in the cloud forest ranged from 0.2 to 2 ng N m-2 s-1. Generally, there was a net deposition in terms of NOx. ¿ American Geophysical Union 1988