Tolbutamide and glibenclamide (glyburide) were administered to normal hamsters, mice or rats in daily doses proportional to their body weight and equivalent to those used in human therapy. The animals were sacrificed after 6 to 8 weeks of treatment. Pieces of pancreas or isolated pancreatic islets were incubated or perifused in a medium containing glucose or tolbutamide, with or without Ieucine-1¹⁴C or glucose-U-¹⁴C. The results indicate that the B cells of sulfonylurea treated animals synthesized and released less insulin and oxidized less glucose than those of insulin or saline treated controls. Accordingly, at least in the glibenclamide treated animals, the tolerance for glucose and the insulinogenic response to a glucose load in vivo were suppressed. Although insular function tended to return, to normal after treatment was discontinued, the results reported in this paper do not support the generally accepted view that the lasting therapeutic effectiveness of the sulfonylureas is due to a beta-cytotrophic action.