Streptozotocin-induced diabetes mellitus is known to impair connective tissue repair in the perforated rat mesentery. The aim of the present investigation was to study quantitatively by morphometrical techniques the influence of diabetes on some aspects of the cellular ultrastructure related to connective tissue contraction in such healing. The cellular volume density increased significantly with time, presumably as a consequence of disappearance of interstitial edema. No difference was found in the amount of healing tissue formed between controls and diabetics. The surface volume density of the plasma membrane was significantly higher in control animals on Days 1-10, indicating an increased number of cellular protrusions and spikes which relate to the motility of the cells. The volume density of contractile filaments did not differ between controls and diabetics. The results suggest a reduced surface density of plasma membrane in diabetic cells, a finding which is compatible with reduced wound contraction in diabetes.