PURPOSE: This experimental study was conducted to determine serial morphological changes of rat's kidney with chronic puromycin aminonucleoside (PAN) nephropathy. Special emphasis was given to the occurrence of glomerular hypertrophy and its relationship to the subsequent development of focal segmental glomerulosclerosis(FSGS). METHODS: Sprague-Dawley rats weighing 200-230g were used and divided into control(n=9) and experimental group(n=15). Rats were given subcutaneous injections of PAN at a dose of of 2mg/100g body weight, or an equivalent volume of normal saline and six injection were given over a period of 9 weeks, at weeks 0, 1, 3, 5, 7 and 9. At weeks 4, 8 and 11, rats were sacrificed and kidney weight, kidney weight/body weight(%) and various laboratory tests including serum protein and albumin were determined. Renal tissues were prepared with Histochoice(R) fixative and paraffin embedding for morphologic study. RESULTS: Kidney weight and kidney weight/body weight(%) were increased significantly in experimental group compared to controls at 4, 8 and 11 weeks. Heavy proteinuria along with lowering of serum protein and albumin and elevation of serum cholesterol was seen in experimental group at week 4 and this change became more marked on weeks 8 and 11. The frequency of FSGS in experimental animal, at week 4, 8 and 11 were 0.6%, 10.6% and 26.2% respectively(p<0.05) and the development of FSGS was more marked in juxtamedullary glomeruli compared to cortical glomeruli. Glomerular surface area showed significant increase in experimental animals compared to controls(p<0.01), the percentage of increase being 12.0, 14.7 and 12.3% at week 4, 8 and 11. And the surface areas of juxtamedullary glomeruli were larger than those of cortical glomeruli throughout the study period. CONCLUSION: In summary, present study indicates that glomerular hypertrophy occurs and precedes the development of FSGS in rats with chronic PAN nephropathy and juxtamedullary glomeruli are more susceptible to developing FSGS compared to cortical glomeuli.