Phenylalkyloxirane carboxylic acids and esters are a new class of potent hypoglycaemic substances. Sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (B 807-27) produces a dose-dependent hypoglycaemic effect when administered orally or intravenously to several fasted laboratory animals, i.e. rats (with and without adrenalectomy), guinea pigs, dogs, streptozotocin-treated diabetic pigs and db/db-mice. In addition, the substances have a more pronounced lowering effect on ketone bodies in the blood than any other known substance. The minimal dose for lowering blood glucose significantly in rats is 15 mumol/kg. The corresponding dose for a significant lowering of ketone bodies in the blood is less than 2.4 mumol/kg. Hence, the substance B 807-27 is approximately five times more potent than tolbutamide and 30 times more potent than the biguanide buformin with respect to lowering blood glucose levels. B 807-27 differs from the sulphonylureas in that it fails to stimulate insulin secretion. In contrast to the biguanides, the substance decreases rather than increases blood lactate concentration and blocks both fatty acid oxidation and gluconeogenesis. The therapeutic usefulness of these compounds in diabetic man remains to be elucidated.