The role of endogenous nitric oxide and sulphydryls in gastric protection afforded by sodium cromoglycate against ethanol-induced gastric lesions was studied in rats. Drugs were administered either intraperitoneally (i.p.) or subcutaneously (s.c.) 30, 45 or 60 min before oral administration of ethanol. Administration of cromoglycate before ethanol dose-dependently inhibited ethanol-induced gastric lesions. Pretreatment with NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide biosynthesis, dose-dependently aggravated gastric lesions and reduced cromoglycate-induced gastric protection. The attenuating effect of L-NAME on gastric protection elicited by cromoglycate was reversible by pretreatment with L-arginine but not by D-arginine. On the other hand, ethanol-induced gastric lesions were found to be associated with a reduction of nonprotein sulphydryl content of glandular stomachs. Pretreatment with cromoglycate prevented non protein sulphydryl depletion and afforded protection. Pretreatment with N-ethylmaleimide, a sulphydryl blocker, caused dose-dependent enhancement of ethanol-induced gastric lesions and further depletion of non protein-sulphydryl. Treatment with N-ethylmaleimide before cromoglycate reduced the gastric protection that was associated with depletion of nonprotein sulphydryls. Furthermore, combined N-ethylmaleimide and L-NAME pretreatment caused a greater aggravation of ethanol-induced gastric lesions and significantly produced a higher reduction of the protective effects of cromoglycate. However, pretreatment with L-arginine only partially restored the protective effects of cromoglycate. These results suggest that the protective effects of cromoglycate may be dependent on the maintenance of a critical level of both endogenous nitric oxide and nonprotein sulphydryls in the gastric mucosa.