We report the observation of substantial emissions of NOx from a several-hundred-square-kilometer region of savanna in northern Namibia in September 1992. The estimated emission rates lie near the high end of the range of values reported from flux chamber studies on various tropical savanna soils and appear to be associated with a light rainfall event which occurred 4 days prior to the observations. If our measurements are typical for this situation, then biogenic emission of NOx following sporadic rainfall events during and at the end of the dry season may be an important regional source rivaling that arising from savanna burning and may have important consequences for regional scale ozone formation.