Studies in vivo indicate that chronic hyperglycemia is deleterious for insulin secretion. We have used an improved islet monolayer culture system to study chronic modulations of B-cell function. Adult rat islets maintained over several weeks on extracellular matrix in the presence of 11.1 mM glucose responded to an acute stimulation with 16.7 mM glucose by a 5- to 8-fold increase in insulin secretion. When cultured in the presence of higher glucose concentrations, the response to an acute glucose stimulus diminished time and dose dependently. In islets desensitized by exposure to 33.3 mM glucose for 1 week, reduction of the glucose level to 11.1 mM reversed the desensitization within 2 weeks. This desensitization was not limited to the glucose stimulus; responses to other nutrient secretagogues, such as glyceraldehyde and α-ketoisocaproic acid, were also reduced. In contrast, responses of insulin secretion to nonnutrient stimulators (tolbutamide and quinine) and amplifiers (isobutylmethylxanthine and carbachol) showed no desensitization in islets exposed to 33.3 mM glucose. Desensitization similar to that caused by 33.3 mM glucose could be induced by 11.1 mM glucose together with 0.1 mM isobutylmethylxanthine. High glucose also caused a time-dependent loss in compact monolayer organization with disruption of cell contacts. Our studies suggest that 1) generation of the reduced insulin response may be related to the prolonged high insulin secretion rate; 2) expression of the functional change is specific to the nutrient stimulus-secretion coupling; and 3) modifications in intercellular contacts may be involved in B-cell desensitization. (Endocrinology129: 2067–2076,1991)