Abstract Propranolol enhanced utilization and generation of ATP in erythrocytes while ouabain suppressed both of them. After incubation of erythrocytes with propranolol, no change was noted in membrane ATPase activity while incubation with ouabain caused suppression of membrane ATPase. The effect of ouabain on erythrocyte ATP and lactate content is compatible with its inhibiting action of membrane ATPase activity. Propranolol may activate ATP-utilizing system(s) in the cytosol, or it may activate membrane ATPase activity only when combined with factor(s) lost during the preparation of the membrane. The above effect of propranolol may be unrelated to the β-adrenergic receptor blocking action since isoproterenol did not affect the erythrocyte ATP or lactate content and it did not counteract propranolol.