OBJECTIVE A reduction in nephron endowment leading to reduced renal filtration surface area has been implicated in the development of hypertension. The aim of this study was to compare glomerular (and thereby nephron) number and renal filtration surface area in young Wistar-Kyoto rats (WKY) with young spontaneously hypertensive rats (SHR), prior to the development of hypertension in this model. METHODS Using unbiased stereological methods the number and size of glomeruli, as well as total renal filtration surface area were determined in perfusion-fixed kidneys of 4-week-old WKY and SHR. RESULTS At 4 weeks of age, in weight-matched animals, there was no significant difference in the number of glomeruli in the kidneys of SHR compared to WKY (28,620 +/- 1,643 and 25,670 +/- 1,263 glomeruli/kidney, respectively). Similarly, there was no difference in mean glomerular volume (SHR: 4.70 +/- 0.31 x 10(-4) mm(3); WKY: 4.28 +/- 0.20 x 10(-4) mm(3)). Surprisingly, total renal filtration surface area was significantly greater in SHR than WKY (3,867 +/- 116 and 3,176 +/- 83 mm(2), respectively). CONCLUSIONS The renal abnormality underlying the development of hypertension in the SHR is not due to inborn deficits in nephron endowment and/or filtration surface area.