Abstract: The effects of isoprenaline (ISP) on cAMP levels in depolarized and non-depolarized mice uteri were studied in vitro. Results showed that ISP could significantly increase the cAMP levels. This action of ISP could be antagonized by propranolol; and in depolarized mice uteri, it was inversely related to the concentration of exogenous Ca²⁺. Moreover, the action of ISP was markedly enhanced in the uteri from reserpinized mice. The above results suggest that the relaxation of mouse uterus induced by ISP may be related to its agonistic action of β-receptor to cause elevation of cAMP levels and that there may be antagonism between Ca²⁺ and cAMP in uterine smooth muscle cells. In addition, the enhancement of the action of ISP after reserpine treatment may be due to hypersensitivity of β-receptor in the myometrium to exogenous β-adrenoceptor agonist.