Hypertensive renal damage starts in the juxtamedullary cortex (JMC) and extends toward the outer cortex (OC) in spontaneously hypertensive rats (SHR). Previously we have showed that the damage is accompanied by increased interstitial fibrosis and changes in the synthesis, deposition and breakdown of collagen. In the present study we investigated the effect of the changed collagen metabolism on collagen fibril structure, and further, the role of growth factors in the regulation of these mechanisms. 60 weeks old SHR and WKY animals with systolic blood pressure at 214±6 mmHg and 135±2 mmHg were used. Transmission electron microscopy was used to examine collagen fibril diameter. Quantitative Reverse Transcriptase PCR was used to quantify mRNA expression for the growth factors in relation to 18S-ribosomal RNA. ELISA was used to quantify growth factor protein expression. Collagen fibril diameter was increased in the JMC of SHR compared to OC, while diameters were not different in WKY. TGF-b1 mRNA was upregulated in the JMC of SHR, but protein expression was not changed. HGF and EGF protein was decreased in the JMC of SHR compared to OC and compared to WKY. PDGF mRNA expression was higher in JMC of SHR compared to OC and compared to WKY. IGF showed reduced expression in JMC of SHR compared to WKY, but higher than in OC in the same strain. In conclusion, renal interstitial fibrosis in SHR is deposited as thicker collagen fibrils. Growth factor expressions are changed in a complex manner with post transcriptional regulation of TGF-b1 and HGF. The down regulation of HGF may play a role in the formation of renal fibrosis.