Laboratory experiments to quantify the global production of NOx ( NO + NO2) in the troposphere due to atmospheric lightning and biogenic activity in soil are presented. These laboratory experiments, as well as other studies, suggest that the global production of NOx by lightning probably ranges between 2 and 20 MT(N)y - of NO and is strongly dependent on the total energy deposited by lightning, a quantity not well-known. In our laboratory experiments, nitrifying micro-organisms in soil were found to be a significant source of both NO and nitrous oxide (N 2O). The measured production ratio of NO to N 2O averaged 2-3 for oxygen partial pressures of 0.5-10%. Extrapolating these laboratory measurements to the global scale, which is somewhat risky, suggests that nitrifying micro-organisms in soil may account for as much as 10 MT(N)y -1 of NO. Additional experiments with denitrifying micro-organisms gave an NO to N 2O production ratio ranging from 2 to 4 for an oxygen partial pressure of 0.5% and a ratio of less than unity for oxygen partial pressures ranging from 1 to 20%. The production of NO and N 2O, normalized with respect to micro-organism number indicates that the production of both NO and N 2O by denitrifying microorganisms is at least an order of magnitude less than production by nitrifying micro-organisms for the microorganisms studied.