To determine renal function throughout development of hypertension in the spontaneously hypertensive rat (SHR) and its normotensive counterpart, the Wistar-Kyoto rat (WKY), renal clearance studies were performed at 2-wk intervals from 4 to 12 wk and again at 16 wk of age in conscious chronically instrumented rats after recovery (4-6 days) from surgery. The data indicate that the critical period for the development of hypertension in SHR was between 4 and 6 wk of age. Mean arterial pressure sharply increased from 107 +/- 5 (n = 6) to 145 +/- 6 mmHg (n = 6) between 4 and 6 wk of age, did not change between 6 and 10 wk of age, and gradually rose between 10 and 16 wk of age to 183 +/- 3 mmHg. In WKYs, blood pressure increased only slightly from 97 +/- 3 mmHg at 4 wk of age (n = 8) to 110 +/- 3 mmHg at 8 wk of age (n = 8), where it remained through adulthood. Glomerular filtration rate (GFR) in the 4-wk-old SHR was significantly decreased compared with WKY (0.94 +/- 0.03 vs. 1.11 +/- 0.04 ml.min-1 x g wet kidney wt-1), and it recovered to normal level and stabilized by 6 wk of age (1.14 +/- 0.04 ml.min-1 x g wet kidney wt-1). Renal blood flow was lower in the SHR only at 4 and 16 wk; it increased with age in both groups. Renal vascular resistance was higher in the SHR at 4 wk and remained elevated throughout the observation period.(ABSTRACT TRUNCATED AT 250 WORDS)