To define the mechanisms involved in the evolution of diabetes in the Zucker diabetic fatty (ZDF) rat, beta-cell mass and replication rates were determined by immunochemistry, point-counting morphometry, and 6-h 5-bromo-2'-deoxyuridine (BrdU) incorporation. The beta-cell mass in 5- to 7-week-old prediabetic ZDF rats (4.3 +/- 0.06 mg) was similar to age-matched insulin-resistant Zucker fatty (ZF) rats (3.7 +/- 0.05 mg) and greater than that in Zucker lean control (ZLC) rats (1.9 +/- 0.3, P < 0.05). At 12 weeks (after diabetes onset), beta-cell mass in the ZDF rats (8.1 +/- 1.7 mg) was significantly lower than the ZF rats (15.7 +/- 1.8 mg). The mass in the ZF rats was significantly greater than in the ZLC rats (4.3 +/- 0.8 mg, P < 0.05). The beta-cell proliferation rate (mean of both time points) was significantly greater in the ZDF rats (0.88 +/- 0.1%) compared with the ZF and ZLC rats (0.53 +/- 0.07%, 0.62 +/- 0.07%, respectively, P < 0.05), yet ZDF rats have a lower beta-cell mass than the ZF rats despite a higher proliferative rate. Morphological evidence of neogenesis and apoptosis is evident in the ZF and ZDF rats. In addition, even at 5-7 weeks a modest defect in insulin secretion per beta-cell unit was found by pancreas perfusion. These studies provide evidence that the expansion of beta-cell mass in response to insulin resistance and insulin secretory defects in diabetic ZDF rats is inadequate. This failure of beta-cell mass expansion in the ZDF rat does not appear to be from a reduction in the rate of beta-cell proliferation or neogenesis, suggesting an increased rate of cell death by apoptosis.