Pancreatic A-cell function in the newly developed Otsuka Long Evans Tokushima Fatty (OLETF) strain of non-insulin-dependent diabetes mellitus (NIDDM) rats was examined in relation to the morphological changes in their islets and the plasma glucagon responses to insulin-induced hypoglycemia and an arginine test by chronological studies in seven male OLETF and seven male non-diabetic control Long Evans Tokushima Otsuka (LETO) rats each at 10, 16 and 24 weeks of age and eight male OLETF rats that were placed in a cage with a wheel for exercising from 5 to 24 weeks of age. The hormonal contents and morphological features of the pancreas of these rats were examined. After iv injection of insulin, the plasma glucagon level rose significantly from the basal level in OLETF rats at 10 weeks old, but little if at all in those of 16 and 24 weeks old. The pancreatic A cells of LETO rats of all age groups responded equally well to glucopenia. The areas under the response curves of plasma glucagon (∑ΔIRG) during the 90 min of insulin-induced hypoglycemia were 14496±7860 vs 9588±3930, 2257±3018 vs 9235±5447 (p<0.05) and 826±985 vs 9707±2510 (p<0.01) ng·min⁻¹·l⁻¹ in OLETF rats vs LETO rats of 10, 16 and 24 weeks old, respectively. The plasma glucagon responses during the arginine test were higher in OLETF rats than in LETO rats at 10 and 16 weeks but not at 24 weeks of age. Exercise-trained OLETF rats of 24 weeks old had normal ability to secrete glucagon from the pancreas in response to glycopenia (∑ΔIRG: 8645±2467 ng·min ⁻¹·l⁻¹). There were no significant differences in the insulin and glucagon contents of the pancreas of young and old OLETF rats. Morphological studies on the pancreas of sedentary OLETF rats of 24 weeks old revealed enlarged, multilobulate, fibrotic islets in which A cells did not occupy a peripheral position but were widely dispersed, whereas in sections of the islets from exercise-trained rats the microstructure and locations of A and B cells appeared normal. These results demonstrated that the pancreatic A-cell response to glucopenia was impaired in old sedentary OLETF rats, probably due to an abnormal A-cell-B-cell morphofunctional relationship.