Abstract: The effects of dl-demethylcoclaurine (DMC) and isoproterenol (ISO) on β-adrenergic receptors (using radioligand binding assay), as well as on adenylate cyclase (AC) activity in turkey erythrocyte membrane Were studied. The results showed that DMC and ISO exhibited concentration dependent stimulating effects on AC activity. These effects were potentiated by GTP and blocked by propranolol. DMC (Ⅰ×10^-5 M) combined with different concentration of ISO (1×10^-9～1×10^-2 M) enhanced AC activity of ISO in low concentration and decreased it in high concentration.The K_D value of DMC and ISO for AC were found to be 17.3±6.4μM and 8.2±5.8μM, respectively. The intrinsic activity of DMC was 0.29+0.03. The K_I of DMC and ISO for β-adrenergic receptor were 3.0 μM and 1.9μM, respectively. These results showed that DMC and ISO had similar affinity for β-adrenergie receptors, but the intrinsic activity of DMC for AC was lower. These findings directly indicate that DMC is a partial agonist of the β-adrenergic receptor.