Increasing evidence supports a role of glomerular cell proliferation in the development of focal or diffuse glomerulosclerosis. This study investigates the chronology and sequence of cellular events that precede glomerulosclerosis in 5/6 nephrectomized rats. Within three days of renal ablation, a phenotypic switch occurred in which some mesangial cells expressed alpha-smooth muscle actin. This was followed by proliferation of mesangial cells, and to a lesser degree endothelial cells from day 5 to week 4 as detected by immunostaining for the proliferating cell nuclear antigen (PCNA). Glomerular cell proliferation was accompanied by increased immunohistochemical expression of PDGF B-chain. In situ hybridization showed no glomerular PDGF B-chain mRNA expression at the induction of proliferation (day 5), and a marked increase between week 1 and 4 in operated rats. In parallel, increased expression of PDGF receptor beta-subunit protein and mRNA was demonstrated by immunohistochemistry and Northern analysis of total glomerular RNA. The onset of glomerular cell proliferation was also associated with mild glomerular platelet accumulation (as defined by 111In-labelled platelet studies) as well as with fibrinogen deposition. Proteinuria, glomerular sclerotic changes, and leukocyte infiltration all followed cell proliferation. The glomerular leukocyte infiltrate consisted of monocytes/macrophages and increased markedly at week 10 in rats with renal ablation. Thus, our results suggest that in the remnant kidney model: 1) proliferation of intrinsic glomerular cells precedes glomerulosclerosis; 2) proliferation may be initiated by degranulating platelets and sustained by PDGF released from intrinsic glomerular cells; and 3) glomerular monocyte/macrophage infiltration occurs after the proliferation, and may possibly contribute to the development of glomerular sclerotic changes.