1. This study investigated the constrictor responsiveness to endothelin-1 (ET-1, 0.1 nM-0.1 microM) of aortic rings (under 10 g resting tension in Krebs solution) from 2- and 6-week streptozotocin (STZ, 60 mg kg-1, i.v.)-induced diabetic rats and vehicle-treated control rats. 2. In aortae from 2- and 6-week STZ-treated rats, and their corresponding controls, removal of endothelium caused leftward shifts of ET-1 concentration-response curves without affecting maximum responses. 3. Maximum responses to ET-1 were reduced in aortae from both 2- and 6-week STZ-treated rats compared to those from control rats. Such reductions were still evident after removal of the endothelium. 4. Decreased responsiveness to ET-1 of aortae from 2-week STZ-treated rats was still evident after chronic treatment with the aldose reductase inhibitor epalrestat, but not after chronic insulin treatment or in aortae bathed in high glucose (30 mM) Krebs solution. 5. Decreased responsiveness to ET-1 of aortae from 6-week STZ-treated rats (compared with those from controls) was still evident after chronic epalrestat treatment and in high glucose Krebs solution. 6. These data suggest that the decreased responsiveness to ET-1 observed in aortae from 2- and 6-week STZ-induced diabetic rats is not due to abnormal activity of the polyol pathway. The altered responsiveness in aortae from 2-week diabetic rats (compared with those from control rats) may possibly be a manifestation of changes (adaptive or otherwise) which occur as a result of high glucose concentrations in vivo.However, in aortae from rats with diabetes of longer duration, other mechanisms may also play a role in the altered responsiveness, since it was no longer reversible by bathing in high glucose Krebs solution.