Duodenal surface epithelial transport of HCO3(-) was measured by direct titration in anesthetized animals. Alkalinization of the lumen occurred in all species, although basal rates varied considerably: rats (approximately 10), cats (approximately 15), pigs (approximately 25), dogs (approximately 25), guinea pigs (approximately 40), and rabbits (approximately 170 mueq.cm-1.h-1). In cats duodenum transported HCO3(-) at a greater basal rate than jejunum (approximately 5 mueq.cm-2.h-1) and developed a higher transmucosal electrical potential difference (PD, lumen negative). Luminal application of 10 mM HCl for 5 min produced a sustained increase in the rate of duodenal HCO3(-) transport that was accompanied by a rise in appearance of E-like prostaglandin immunoreactivity in the lumen and a decrease in DNA release. In cats pretreated with indomethacin (10 mg/kg iv), acid caused only a transient increase in HCO3(-) transport. Exogenous prostaglandin E2 (1-12 microM, luminal) increased basal HCO3(-) transport in cats, rats, and dogs but had no effect on this transport in guinea pigs and rabbits. However, prostaglandin E2 increased HCO3(-) transport and PD in guinea pigs pretreated with inhibitors of tissue cyclooxygenase activity (indomethacin or aspirin) or gastric H+ secretion (cimetidine). Thus the continuous exposure of the duodenum of herbivores to HCl discharged from the stomach may itself stimulate HCO3(-) transport via an increase in endogenous prostaglandin levels and render exogenous prostaglandins ineffective. Secretin (1-15 CU/kg iv) was without effect in both cats and guinea pigs. In guinea pigs, intravenous glucagon (120-360 micrograms.kg-1.h-1) or gastric inhibitory peptide (5 micrograms/kg) both increased HCO3(-) transport but not PD. Hence, prostaglandin-stimulated and hormone-stimulated mechanisms of HCO3(-) transport probably occur in mammalian duodenum as found previously in the isolated amphibian duodenum. The results suggest that epithelial HCO3(-) transport is a major mechanism of acid disposal, and thus mucosal protection, in mammalian duodenum under the control of hormones and endogenous prostaglandins.