C-fos protein expression was investigated in the nucleus of the solitary tract (NTS) in response to increasing cholecystokinin (CCK) doses and food intake in rats by counting the number of c-fos protein positive cells in the NTS. C-fos protein expression in the NTS dose-dependently increased in response to CCK, the lowest effective dose being 0.1 microg/kg. The ED(50) for c-fos protein expression in the NTS in response to CCK was calculated to be 0.5 to 1.8 microg/kg, depending on the anatomical level of the NTS investigated. Food intake increased c-fos protein expression in the NTS, the maximum number of c-fos protein positive cells being reached at 90 min after the start of food intake. Regression analysis identified a positive correlation between c-fos protein expression and the amount of food intake. Our data indicate that subpopulations of the NTS that are activated by CCK or food intake are involved into the short-term regulation of food intake and the neural control of feeding by the caudal brainstem.