Total parenteral nutrition (TPN) has been shown to alter the serum concentration of certain gastrointestinal hormones. However, the effects of surgical and nutritional techniques have not been evaluated adequately. This study examined the effects of those techniques on serum gastrin and secretin levels in Fischer 344 rats fed TPN. After aseptic superior vena cava cannulation and a 4-day postoperative recovery period with ad libitum access to food and water, 18 rats were randomly and equally (n = 6) assigned to either (1) a baseline group, killed after the recovery period; (2) a TPN group fed sufficient nutrients (67% dextrose, 21.7% amino acid, 11.3% fat) and kilocalories (1.45 kcal/mL) for normal growth and development; or (3) an orally fed (OF) group fed ad libitum a powdered diet isocaloric and isonitrogenous to the TPN solution. Both the TPN and OF groups were fed for 5 days and then killed. Another set of six rats without surgery served as a control (CON) group and had the same diet protocol as the OF group. Serum was analyzed for gastrin and secretin by radioimmunoassay. The OF group had significantly (p < .01) lower gastrin levels than the CON group. Gastrin levels of OF rats were twofold lower (p < .04) than those of the TPN rats and were threefold greater (p < .02) than those of the baseline group. Secretin levels in OF rats were twice as high (p < .02) as those in CON rats. The OF group showed a trend toward significantly (p < .09) higher serum secretin levels when compared with the TPN group.(ABSTRACT TRUNCATED AT 250 WORDS)