This study uses complementary DNA probes to find out if epidermal growth factor (EGF) and EGF receptor genes are activated in healing colonic anastomoses in rats. 35 rats had a colocolonic anastomosis which was removed for study 1, 2, 3, 5, 7, or 14 days after operation. Six animals without surgery served as controls. The specimens were examined by northern blotting, in situ hybridisation, and conventional microscopy. Microscopic healing progressed normally. The expression of the EGF gene was minimal and surgery did not activate it. However, surgical trauma did increase the expression of the EGF receptor gene by 2.2 times when compared to day one. In situ hybridisations localised a strong EGF receptor expression in mucosal epithelial cells in all specimens, and a moderate reaction in fibroblasts in the repair tissue in the anastomotic line. The enhanced EGF receptor gene expression suggests that anastomotic healing is associated with the presence of EGF or an EGF-like substance and its activity increases during the first postoperative week.