Dietary sodium restriction (9 mumol/g food) was started in 4-week-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY), and was continued for 3 or 6 weeks. Low sodium intake for 3 weeks prevented the development of hypertension in SHR. More prolonged restriction caused hypotensive blood pressure levels in both SHR and WKY. At 7 weeks of age, SHR and WKY on a control diet showed similar blood pressure decreases after prazosin; at 10 weeks the response was significantly larger in SHR. Sodium restriction from 4 to 7 weeks of age did not inhibit the prazosin effect, whereas after more prolonged restriction the response was significantly inhibited in SHR compared with control SHR as well as with sodium-restricted WKY. These results indicate that sodium restriction may affect blood pressure in several ways, and that inhibition of the alpha 1-receptor mediated pressor effect is specific to SHR.