Glibenclamide or HB 419 is a new, particularly active hypoglycaemic sulphonylurea. In the normal, conscious dog, HB 419 administered intravenously was 240 or 440 times more active than tolbutamide, depending on whether the dosage is expressed in grams or in moles. The hypoglycaemic effect did not occur in the totally pancreatectomized dog. HB 419 stimulated insulin secretion. HB 419 administered intravenously in the anaesthetized dog increased insulinaemia in the pancreatico-duodenal vein for 7 h, which represents the duration of the experiment. After oral administration of 2 mg/kg, the increase of peripheral insulinaemia persisted for 24 h. Hyperinsulinaemia was also found in the pancreaticoduodenal vein in the conscious dog, when a catheter was permanently inserted into the said vein. The direct insulinsecreting effect has been shown on the isolated, perfused rat pancreas in vitro. This drug also counteracted the inhibitory effect of diazoxide on insulin-secretion both in vivo and in vitro. - Furthermore, this drug potentiated the effects of insulin in the pancreatectomized dog. © 1969 Springer-Verlag.