The present study was designed to evaluate the renal response to atrial natriuretic factor (ANF) in young rats developing spontaneous hypertension (SHR) and compare this response to age-matched, normotensive controls (WKY) and adult animals. At 6 weeks of age, intravenous infusion of ANF (0.25 μg/kg min) in anesthetized, euvolemic rats produced a significantly larger natriuresis and diuresis in SHR compared with WKY rats; this strain difference was not observed in rats 11 weeks of age. SHR showed no age-related change in the natriuretic response to ANF, whereas adult WKY rats exhibited a greater response than young WKY rats. To determine the effect of renal perfusion pressure on the magnitude of the renal response to ANF, additional groups of 6- and 11-week-old SHR were studied while renal perfusion pressure was lowered acutely by aortic constriction (SHR-AC) to values similar to age-matched WKY rats. In young rats, the diuretic and natriuretic response to ANF was greatest in SHR, intermediate in SHR-AC, and lowest in WKY rats. In adult animals, the natriuretic and diuretic response was similar in SHR and WKY rats and tended to be less in SHR-AC. These results in both 6- and 11-week-old SHR are consistent with previous reports that the magnitude of the response to ANF is directly related to acute changes in renal perfusion pressure. Furthermore, we conclude that 1) the kidney of young SHR is hyperresponsive to ANF, 2) WKY rats, but not SHR, exhibit age-related increases in responsiveness to ANF, 3) increased renal perfusion pressure cannot completely account for the exaggerated renal response in young SHR, and 4) the relation between pressure and the response to ANF is not observed with chronic elevations in renal perfusion pressure in hypertensive rats.