SummaryIllustrated by two personal cases, the authors discuss the interest of simultaneous determinations of blood glucose and plasma insulin for the diagnosis of pancreatic islet-cells tumors in several conditions : basal determinations, prolonged starvation and pharmacological tests using tolbutamide or glucagon. They report the interest of diazoxide in the management of these patients and present data indicating that this drug acts, at least in part, by a decrease in insulin secretion. In one case, they have studied the insulin content of the tumor and of the surrounding pancreas. They report the results of in vitro incubation of fragments of the tumor in the presence of glucose, at two different concentrations, and in the presence of glucose (too mg/ioo ml) plus tolbutamide (100 /µg/ml) or glucagon (5 /µg/ml).