Profiles of the mixing ratio of NOx(NOx = NO+NO2) for the clean troposphere are presented. The NOx mixing ratio increases from 10 pptv in surface air to values of about 200 pptv at the tropopause. The profile of NOx is governed by intrusion of odd nitrogen from the stratosphere into the troposphere. Anthropogenically produced NOx from ground level sources influences the NOx profile only in the lower troposphere.