Conflicting reports exist about the effects of diabetes on vascular function. In the present study we investigated (1) the influences of diverse stages of diabetes on mechanical activity and pharmacological reactivity of portal vein and tail ventral artery isolated from male rats 7, 21 or 30 days after alloxan injection (150-180 mg/kg) and (2) the effects of in vitro or in vivo insulin treatment. Various parameters were used to assess the diabetic state (serum glucose levels, body weight, percentage of glycosylated hemoglobin and glucosuria). Isometric developed tension of portal vein from control rats was 10.86 +/- 0.41 mN (n = 54), and was enhanced significantly in diabetics (+56% at 21 days and +45% at 30 days; p less than 0.001 vs. controls). When challenged with noradrenaline, portal veins from diabetics exhibited a greater contractility and lower reactivity (as reflected by EC50 values). The magnitude of responses to KCl remained similar to those obtained in controls, but nonetheless the reactivity seems to be higher. Tail ventral artery from diabetics also exhibits a greater contractility in response to noradrenaline with no significant changes in EC50 values. The results demonstrate that diabetes affects mechanical performance of the vascular smooth muscle in a differential manner depending on the stage of the endocrinopathy and on the types of vessel studied. These modifications were not avoided by insulin treatment.