This study displayed the physiological effects the tricyclic antidepressants amitriptyline or trimipramine have on glucose homoeostasis in male Wistar rats. An insulin secreting cell line (INS-1) was also used to determine effects tricyclic antidepressants have on insulin secretion and insulin displacement. Thirty rats each received a 1 mg kg−1 dose of amitriptyline or trimipramine for a period of 14 weeks; another 14 rats served as the control group. Blood glucose, serum insulin and muscle and liver glycogen levels were determined. Kidney, liver and muscle insulin degradation was measured and compared with insulin degrading enzyme concentrations in the latter two tissues. INS-1 cells were used to determine the effect 1μM amitriptyline has on insulin secretion. Displacement studies for [3H]glibenclamide by amitriptyline or trimipramine were undertaken on INS-1 cells. A significant increase in blood glucose (P < 0.01) was found for both test groups after 6 and 14 weeks of receiving the medication, which may be related to a significant decrease in liver and muscle glycogen levels (P < 0.001). Serum insulin levels remained unchanged, although a significant increase in insulin degradation was observed in the muscle, liver and kidney, which may be related to a significant increase in insulin degrading enzyme (P < 0.001) that was found. A significant increase in insulin secretion was observed for the INS-1 cells treated with amitriptyline, although no significant displacement for the [3H]glibenclamide was evident for amitriptyline or trimipramine. The significant alterations in glucose homoeostasis observed, as well as the significant changes associated with insulin secretion and degradation associated with amitriptyline or trimipramine treatment, imply that prolonged use of these medicines may lead to insulin resistance and full blown diabetes.