The effect of human epidermal growth factor (hEGF) on gastric epithelial proliferation was studied in vitro and in vivo. In the first study, subconfluent gastric epithelial cells derived from fetal rabbit stomach were incubated with hEGF at doses of 0, 1, 10, 50, 100, 500 ng/ml and tritiated thymidine incorporation into DNA was determined. In the second study, fasted rats were given intraperitoneal injections of 100 micrograms/kg hEGF and ornithine decarboxylase activity was quantitated. In the third study, hEGF at doses of 0, 5, 10, 25, 50 micrograms/kg was administered perorally 3 times a day for 7 days. During the experiment, rats were allowed ad libitum intake of food and water. After 7 days, all rats were injected with tritiated thymidine 1 microCi/g body weight and sacrificed 1 h later. Sections from fundic and antral mucosae were processed for autoradiography. While hEGF stimulated tritiated thymidine incorporation into DNA by gastric epithelial cells in vitro, neither parenteral nor peroral administration of hEGF affected the proliferative measurements as assessed by ornithine decarboxylase activity or autoradiography in adult rat gastric fundic and antral mucosae in vivo. These results indicate that although hEGF has a direct proliferative effect on fetal rabbit gastric epithelial cells, exogenously administered hEGF has little effect on adult rat gastric mucosal steady state cell proliferation under physiological conditions.