This study describes immunofluorescence microscopy (IFM) of kidneys of untreated (or saline-treated) spontaneously hypertensive rats (SHR) and compares the findings with those in age-matched, untreated normotensive Wistar Kyoto rats (WKY), and heparin treated (150 units aqueous heparin subcutaneously twice daily for 30 days) SHR. The rats varied in age from 30 to 112 weeks (before treatment) and were divided into young (< 52 weeks) and old (⩾ 52 weeks) groups. Separate sections from the kidneys of all rats were stained with fluorescein-isothiocyanate-conjugated rabbit antisera against rat γ-G (IgG, IgM, and IgA), IgG, fibrinogen (Fib), third component of complement (C 3), and albumin. The kidneys were evaluated also by light microscopy (LM) in all of the rats and by electron microscopy (EM) in half of the rats. A semiquantitative assessment of IFM findings on a 0 to 4+ scale and scoring of LM findings on a 0 to 39 scale were made. The glomerular lesions were more striking and uniform than the arterial vascular lesions in old SHR. The glomeruli of old SHR (av: 93 weeks) were positive for all the fluorescence materials except albumin. Fib was most marked (3+ to 4+) involving 50 to 75% of the glomeruli in 75% untreated (or saline treated) old SHR. Thrombi were found in the glomeruli also by LM and EM (large amounts of fibrin and platelet aggregates). The arterial vessels were unimpressive by IFM with the exception of those with arteritis in which 3+ to 4+ Fib was found. These findings in untreated (or saline treated) old SHR contrast with the inconspicuous LM, EM, and IFM findings in young SHR and WKY of all ages. Heparin-treated old SHR had significant ( P < 0.05) decreases of glomerular fibrinogen (IFM), percentage glomeruli with thrombi (LM), and absence of fibrin and platelet aggregates in the glomeruli studied by EM. In these old SHR, the mean arterial pressure (MAP) was significantly ( P < 0.05) lower than the MAP in untreated old SHR. Thus, this study indicates that glomerular thrombosis is a significant renal lesion in old SHR and heparin can alter the glomerular thrombosis and arterial blood pressure. Further study, however, would be necessary to elucidate the mechanisms of the above beneficial effects of heparin.