In dogs with a direct pancreatic fistula, a duodenal cannula and a catheter in a saphenous vein, plasma secretin levels, changes in the flow, bicarbonate and chloride concentrations of the exocrine pancreatic secretion as well as in the pH of intraduodenal content have been studied 12 hours after the ingestion of a standard diet. Under these conditions the pancreatic secretion showed a biphasic response with a maximum flow and bicarbonate concentration during the 0-4 and 8-12 h postprandial periods. This coincided with a marked decrease of pH in the intraduodenal content, with values close to 4.5. On the other hand, during the first postprandial hour, plasma secretin values increased from basal ones (218.66 +/- 27 pg/ml) to 448.94 +/- 66 pg/ml, remaining elevated for four hours after the meal ingestion. However, no increase occurred between 8-12 h when intraduodenal pH reached values below 4.5. This study indicated that: 1) plasma secretin levels increased significantly (P less than 0.05) after the ingestion of a standard solid diet, and 2) determinants liberating secretin were not only the presence of a duodenal pH below 4.5, but probably the presence of some macronutrients from the meal.