The effect of high dietary sodium (8%) on blood pressure in spontaneously hypertensive (SHR), borderline hypertensive (BHR), and normotensive Wistar-Kyoto (WKY) rats was determined weekly by tail cuff plethysmography for one week of baseline and four weeks of diet. After 4 weeks, significant elevations in systolic blood pressure were found in SHR and BHR groups, but not in WKY. BHR studied an additional 4 weeks showed a further progression of hypertension, reaching levels nearly equal to control SHR. Direct measurement of arterial pressure in conscious animals in their home cage confirmed the elevation in pressure in both SHR and BHR groups. Metabolic studies revealed that the high sodium diet reduced body weight in SHR and BHR strains, but not in WKY. Although both urinary volumes and sodium excretion values were significantly lower in SHR and BHR compared with WKY, this effect disappeared when adjustments for body weight were made. Plasma norepinephrine determinations revealed a significant response to cold stress in all groups. Plasma epinephrine was elevated in all strains in response to cold stress; however, a consistent statistical elevation was seen only in WKY. The BHR is discussed as a model for determining the triggers responsible for environmentally-induced hypertension.