The role of extracellular calcium and high blood pressure stress in altered vascular adrenergic responsiveness in rings of femoral artery from spontaneously hypertensive rats (SHR) was investigated. A model in which partial ligation of the external iliac artery prevents the increase in blood pressure to the ipsilateral femoral artery was used to assess the effect of the increase in pressure stress on these alterations. Age-matched (5-week-old) male Wistar-Kyoto rats (WKY) and SHR were used in the study. Partial ligation was performed before a substantial increase in blood pressure occurred (6 weeks of age), and studies on vascular reactivity were carried out at 10 to 12 weeks of age when the SHR were considered hypertensive (indirect systolic blood pressure greater than 150 mm Hg). Maximal contractility of rings of unprotected femoral artery from the SHR in response to KCl in either a normal (2.5 mM) or low (0.25 mM) calcium Krebs solution was significantly greater (p less than 0.05) than was that of protected vessels from the SHR or protected and unprotected vessels from the WKY; however, no difference in the sensitivity to KCl was observed. Isoproterenol-induced relaxation was significantly attenuated in rings of vascular smooth muscle from unprotected femoral arteries of the SHR (p less than 0.05), while the responses of protected vessels from the SHR were similar to controls. Equilibration of vascular smooth muscle in a low calcium Krebs solution resulted in an increase in beta-adrenergic mediated relaxation in all groups.(ABSTRACT TRUNCATED AT 250 WORDS)