The influence of standard heparin or low molecular weight (LMW) heparin on healing of abdominal wounds and colonic anastomoses was studied in rats. Subcutaneous injection of 1 XaI U/g b.w. of standard or LMW-heparin or 0.5 ml physiologic saline was given 12 hours preoperatively and daily for 3 or 7 days postoperatively. Breaking strength of the abdominal wound and the anastomosis was measured, as were haemoglobin and albumin in serum. Hydroxyproline as a measure of collagen and tissue dry weight was determined in standardized segments of colonic wall adjacent to the anastomosis. Except for significant increase in breaking strength of the anastomosis after 7-day injection of LMW heparin, no differences in the parameters of wound healing were found after 3 or 7 days. In rats receiving standard heparin there was increased bleeding tendency (reduced haemoglobin) compared with the LMW-heparin group and the controls. The administered heparin thus did not negatively influence healing, and standard and LMW-heparin did not differ in this respect