Nitric oxide (NO) is synthesized by most regions of the gastrointestinal tract and is an important regulator of mucosal function and integrity. In this study we examined the ontogenic appearance of constitutively expressed Ca2+ -dependent NO synthase (cNOS) and inducible Ca2+-independent NO synthase (iNOS) activity, in the colon of neonatal rats. Furthermore, the susceptibility of the colon to damage after induction of iNOS activity following bacterial endotoxin treatment was also examined. Segments of distal colon were removed from either control rat pups (aged between 10 and 25 d) or from animals pretreated with the following agents: 1) Escherichia coli lipopolysaccharide [LPS; 3 mg/kg, intraperitoneally (i.p.), 4 h before sacrifice], 2) dexamethasone (2 mg/kg, i.p., 1 h before administration of LPS) or aminoguanidine (25 mg/kg, i.p., at the same time as LPS). NOS activity was measured via the conversion of L-[14Clarginine to L- [14C]citrulline. Samples of colon were assessed for damage by either light microscopy or by measurement of the malondialdehyde content to estimate lipid peroxidation. In untreated animals cNOS activity increased during the first 20 postnatal days and fell postweaning at 25 d. LPS treatment resulted in a significant increase in iNOS activity in all age groups examined, with maximal activity occurring between 10 and 15 d of age. This coincided with the greatest histologic damage score and lipid peroxidation. Dexamethasone or aminoguanidine attenuated the effects of LPS suggesting the involvement of iNOS in these responses. These data suggest that colonic eNOS activity in the neonatal rat may be important during development and maturation of that tissue. Furthermore, the colon of the preweaned rat is more susceptible to the detrimental effects of LPS-induced NO production than the colon of postweaned animals.