The distribution of Ca(2+)-dependent and Ca(2+)-independent nitric oxide synthase (NOS) was studied in rabbits and in control and endotoxin-treated rats and guinea-pigs. There was a widespread localization of NOS which differed for the two forms of the enzyme and which showed marked differences between species. Endotoxin induced the activity of the Ca(2+)-independent NOS in many tissues and also increased the activity of Ca(2+)-dependent NOS in the rat ileum and caecum. These results demonstrate the differential distribution of NOSs in control and endotoxin-treated animals and emphasize the widespread biological role of nitric oxide (NO).