Adult, male Sprague-Dawley rats were randomly divided into the following groups: control, control + insulin, alloxan-diabetic, and diabetic + insulin. Diabetes was induced by a single dose of alloxan (200 mg/Kg body weight), administered i.p. in isotonic saline. Rats in the control + insulin group received 3 I.U. of insulin daily. The diabetic + insulin group were given sufficient insulin (3–5 I.U.) to maintain their urine glucose levels in a 0-1/4 % range. Urinary glucose levels for each animal were determined daily. The animals were killed after 21 days and the fasting blood glucose concentration, serum and 24 hour urinary levels of hydroxyproline, gingival collagen and protein concentrations were determined. Collagenolytic activity of rat gingivae in tissue culture was also measured. The alloxan-diabetic rats showed (1) a marked hyperglycemia and glycosuria, (2) decreased levels of water-soluble protein and acid-soluble collagen in gingivae, (3) an increased collagenase activity of gingival fragments in culture and (4) a marked increase in the serum concentration and urinary excretion of hydroxyproline. The administration of insulin to the diabetic rats partially or completely returned to normal levels each of the altered physiological parameters described above. These insulin sensitive changes appear, therefore, to have resulted from the diabetic condition of the rats rather than from any non-diabetic effects of alloxan treatment. Similar changes in gingival collagen metabolism in humans, if present, could be related to the more rapid periodontal destruction observed in diabetic patients.