Membrane Ca(2+)-ATPase activity was stimulated in vitro separately by T4 (10(-10) M) and by epinephrine (10(-6) M). In the presence of a fixed concentration of T4, additions of 10(-8) and 10(-6) M epinephrine reduced the T4 effect on the enzyme. beta-Adrenergic blockade with propranolol (10(-6) M) prevented stimulation by epinephrine of Ca(2+)-ATPase activity, but did not prevent the suppressive action of epinephrine on T4-stimulable Ca(2+)-ATPase. In contrast, alpha 1-adrenergic blockade with unlabelled prazosin restored the effect of T4 on Ca(2+)-ATPase activity in the presence of epinephrine. Like propranolol, prazosin prevented enhancement of enzyme activity by epinephrine in the absence of thyroid hormone. Neither prazosin nor propranolol had any effect on the stimulation by T4 of red cell Ca(2+)-ATPase in the absence of epinephrine. Analysis of radiolabelled prazosin binding to human red cell membranes revealed the presence of a single class of high-affinity binding sites (Kd, 1.2 x 10(-8) M; Bmax, 847 fmol/mg membrane protein). Thus, the human erythrocyte membrane contains alpha 1-adrenergic receptor sites that are capable of regulating Ca(2+)-ATPase activity.