Background/Aims: Cross-transplantation studies between animals with genetic hypertension and normotensive animals indicate a key role of the kidney in development of hypertension, and studies in young spontaneously hypertensive rats (SHR) have shown reduced glomerular filtration rate (GFR) and renal blood flow (RBF) for a short period at the age of 4–6 weeks during blood pressure increase. We tested the hypothesis that a decline in GFR during development of hypertension in SHR might be more pronounced in juxtamedullary cortex than other cortical zones. Methods: By use of the aprotinin method, total and zonal cortical GFR was measured in anaesthetized Wistar-Kyoto (WKY) rats and SHR at the ages of 2, 4, 6, 8 and 10 weeks. RBF was measured by a transit time flowmeter. Results: Body and kidney weights in SHR and WKY were not significantly different in any age group (p >0.05). Mean arterial blood pressure (MAP) was not different at the age of 2 weeks (79 ± 6 mm Hg in SHR and 74 ± 5 mm Hg in WKY, p > 0.05), but was significantly higher in 4-week-old SHR (104 ± 1 mm Hg) compared to 4-week-old WKY (77 ± 3 mm Hg) (p < 0.01). The difference in blood pressure increased with age from 4 to 10 weeks. RBF, total GFR, and outer, middle, and inner cortical GFR increased with age but were not different in SHR and WKY in any age group (p >0.05). Renal vascular resistance was increased from 4 weeks of age in SHR (21.5 ± 1.8), significantly higher than WKY (14.4 ± 0.9 mm Hg·ml<sup>–1</sup>·min·g) (p < 0.01) and stayed at higher values in older age groups (p ≤ 0.01). Conclusion: RBF, total and zonal GFR are not significantly different in anaesthetized SHR compared to WKY at ages from 2 to 10 weeks and GFR in juxtamedullary cortex is not decreased in SHR during onset of hypertension. The results from the present study indicate that development of hypertension cannot be explained by a temporary decline in RBF or total or zonal GFR.