The influence of tolbutamide (20 mg/kg and 100 mg/kg i.v.) on blood flow to the entire pancreatic gland and to its endocrine tissue was studied in anaesthetized rats. Non-radioactive microspheres 8.8 micron diameter were used for determination of local blood flow rates. The islet tissue was subsequently stained intravitally with dithizone. The organ was subdivided into small samples, frozen and thawed then scanned in the microscope for spheres in endocrine and exocrine tissue, respectively. Tolbutamide only insignificantly affected total organ blood flow (0.53 ml X min-1 X g-1, controls; 0.61 ml X min-1 X g-1 lower dose; 0.39 ml X min-1 X g-1 higher dose) but significantly increased the percent islet perfusion (3.0% controls; 6.6% lower dose; 10.5% higher dose of tolbutamide). The results favour the view that these vascular effects might facilitate insulin release into the systemic circulation.