Background/Aims: Angiotensin II (Ang II) mediates progressive nephron loss in diabetes and stimulates apoptotic cell death in several tissues. We studied the extent of apoptosis in streptozotocin (STZ) induced diabetic nephropathy in the rat and the effects of insulin and type 1 (AT<sub>1</sub>) or type 2 (AT<sub>2</sub>) Ang II receptor blockade with losartan or PD123319, respectively. Methods: Three groups of rats were studied after 2 and 12 weeks: (1) controls ; (2) STZ-diabetic rats (STZ rats), and (3) STZ-diabetic rats with insulin implants. Additional rats were treated with losartan (25 mg/kg/day) and/or PD123319 (10 mg/kg/day) for 2 weeks. Kidneys were examined for apoptosis, using the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay, DNA laddering, and electron microscopy. Immunoblotting determined expression of the proapoptotic protein Bax and of the antiapoptotic protein Bcl-2 in proximal tubules. Results: Diabetes caused a significant increase in apoptosis, involving tubular and interstitial cells of cortex and medulla, but not glomerular cells (2 weeks: controls 264 ± 94 vs. STZ rats 1,501 ± 471 apoptotic nuclei/kidney section; p < 0.02; n = 6–8), an effect reversed by insulin. In STZ rats, ultrastructural examination revealed chromatin condensation and nuclear fragmentation in tubular and interstitial cells. At 2 and 12 weeks, a significant decrease in the expression of the antiapoptotic protein Bcl-2 occurred in STZ rat proximal tubules, with restoration by insulin. In STZ rats, treatment for 2 weeks with losartan or PD123319 inhibited apoptosis in the kidneys, with no additive effect of the combination therapy. Conclusions: Apoptosis occurs in diabetic nephropathy, involving tubular and interstitial cells, an effect reversed by insulin therapy. Furthermore, the effects of AT<sub>1</sub> or AT<sub>2</sub> receptor blockade suggest that Ang II is involved in mediating apoptosis in the diabetic kidney.