To define the vagal circuitry mediating the cephalic phase insulin response (CPIR), this reflex was measured in conscious, freely moving rats that had previously undergone selective abdominal vagotomies that spared different columnar subpopulations of dorsal motor nucleus of the vagus (dmnX) neurons. The CPIR was defined as an increase of plasma insulin from basal at 2 min after the start of ingestion. The CPIR measured in peripheral blood after chow ingestion was reliable and significant (P less than 0.05) in rats with all branches intact, +24.9 +/- 5.1 microU/ml (+130% increase from basal); rats with only the two gastric branches and the hepatic branch intact, +27.0 +/- 3.5 microU/ml (+153%); and rats with only the hepatic branch intact, +13.5 +/- 4.8 microU/ml (+188%). No significant response occurred in animals with only the two celiac branches intact, +1.8 +/- 1.8 microU/ml (+15%) or in those with none of the branches intact, +3.9 +/- 3.3 microU/ml (+21%). The CPIRs measured in portal vein blood were generally larger but showed the same pattern across groups. Plasma glucose measurements of portal vein blood indicated that with chow ingestion no significant absorption had occurred by 2 min, whereas with either milk or glucose intake absorption did occur. Subsequent bilateral electrical cervical vagal stimulation-induced insulin and glucagon responses in the same animals under anesthesia showed the same branch dependency. It is concluded that the CPIR is mediated by the two gastric and the hepatic branches but not the two celiac vagal branches. The perikarya of the preganglionics innervating the pancreatic B-cells are contained within a large pool occupying the two medial columns of the dmnX.