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The Goto-Kakizaki (GK) rat represents a spontaneous animal model of non-insulin-dependent diabetes mellitus (NIDDM) characterized by impaired glucose-stimulated insulin release from the pancreatic β cells. To study whether an alteration in their islet β-cell numbers occurs in parallel with the impairment of insulin secretion in this model, the relative volume density of β cells was determined by means of conventional point sampling in immunostained 4-μm-thick sections of the pancreata from 8-week-old GK rats. The pancreata of nondiabetic Wistar rats were used as control parenchyma. In the GK pancreata the majority of islets was found to have a normal structure; only a few of the islets demonstrated an irregular shape (starfish-shaped islets) with fibrosis. The relative volume of the total endocrine parenchyma was found to be 2.0 ± 0.6% (mean ± SEM) of the whole pancreatic parenchyma in GK rats. In the control rats the corresponding value was 2.3 ± 0.5%. The islet β-cell density was also similar in GK and control rat islets, amounting to 15.2 ± 8.5 and 66.9 ± 6.6%, respectively. Thus, the total relative volume of β cells was 1.5 ± 0.5% in GK rats and 1.6 ± 0.4% in controls. In conclusion, the density of β cells is preserved in the pancreata of the young, diabetic GK rats, suggesting the absence of a causal relationship between the relative pancreatic β-cell volume and the impaired glucose-induced insulin secretion in this NIDDM animal model.