Male Wistar rats were subjected to colonic resection and randomized to one of four groups: control group (intraperitoneal NaCl, intravenous NaCl); 5-fluorouracil (5-FU) group (intraperitoneal 5-FU, intravenous NaCl); folinic acid group (intraperitoneal NaCl, intravenous folinic acid); and 5-FU-folinic acid group (intraperitoneal 5-FU, intravenous folinic acid). Treatment was started immediately after surgery and continued until the animals were killed at 3 or 7 days. Anastomotic complications (abscesses or dehiscence) occurred in four of 33 animals in the control group, 12 of 36 in the 5-FU group, one of 32 in the folinic acid group and nine of 36 in the 5-FU-folinic acid group. Anastomotic and skin breaking strength did not differ between groups on day 3 but by day 7 were significantly reduced in the 5-FU group. In rats given 5-FU-folinic acid, breaking strength was also reduced, but less so than in the 5-FU group. Breaking strength in animals receiving folinic acid was similar to that in the control group. In this model colonic healing was impaired after intraperitoneal 5-FU administration, but when folinic acid was added no further deterioration occurred.