The effect of growth hormone on the bursting strength of left colonic anastomoses was investigated experimentally. Seventy-two 3 month-old female rats were randomized into two groups receiving daily injection of either saline (controls) or 2.0 mg biosynthetic human growth hormone per kg body weight per day. All injections were started 7 days before a left colonic resection and anastomosis, and continued until sacrifice at the 2nd, 4th or 6th post-operative day. The bursting strength of the anastomoses was tested in anaesthetized, living rats. The bursting pressure and the bursting wall tension of the growth hormone treated animals were increased two-fold on the second post-operative day (2p less than 0.005) and three-fold on the fourth post-operative day (2p less than 0.05), compared with controls. There was no difference in the bursting pressure or the bursting wall tension on the sixth postoperative day. The mass of the resected segment was increased by the pre-operative growth hormone treatment, whereas the hydroxyproline content was unchanged. In conclusion, treatment with biosynthetic human growth hormone increases the strength of colonic anastomoses in the early phases of healing.