The effects of graded duodenal acidification on plasma concentration of immunoreactive secretin, and motilin in portal vein (PV) and carotid artery (CA) were investigated in 6 anaesthetised pigs in which the proximal duodenum was excluded and sequentially irrigated with isotonic saline (pH 7.0) and with hydrochloric acid (HCl) delivered at successive rates of 2, 8 and 40 mM H+/40 min (pH of 2.8, 1.9 and 1.0, respectively) under a constant flow of 10 ml/min. The release of secretin was first observed at pH 1.9 (from basal 4.2 +/- 0.3 to a peak of 26.6 +/- 9.8 pM in PV and from 3.4 +/- 0.3 to 6.8 +/- 0.9 pM in CA) and further increased at pH 1.0 (peaks of 60.9 +/- 16.3 in PV and 16.8 +/- 2.6 pM in CA). In contrast, only the highest HCl concentration (pH of effluent: 1.0) induced significant increases of plasma motilin (PV: peak of 25.2 +/- 4.9 for basal 13.3 +/- 1.1 pM, CA: 14.3 +/- 2.4 for basal 10.5 +/- 0.6 pM). A sharp decrease of the plasma secretin and motilin concentration was observed when the venous drainage of the duodenal segment was occluded, followed by a rapid increase when the clamp was released. In the present experimental conditions, duodenal motilin-producing sites were less sensitive to graded luminal acidification than secretin cells. Thus, the release of duodenal motilin in response to variations of luminal pH may be expected to occur primarily from the most proximal part of the duodenum in physiological conditions.