We examined the basal adenosine 3′,5′-cyclic monophosphate (cAMP) levels and forskolin-induced cAMP accumulation in cultured Chinese hamster ovary cells (CHO) expressing different levels of human β₂-adrenoceptors. Both the basal and forskolin-induced cAMP accumulation in the cells that express higher density of β₂-adrenoceptors (CHO-β₂/H; 560 fmol/mg protein) were larger than those in the cells that express lower density of β₂-adrenoceptors (CHO-β₂/L; 270 fmol/mg protein). In addition, isoproterenol-induced cAMP accumulation was also augmented as the number of β₂-adrenoceptors was increased. ICI 118,551, a selective β₂-adrenoceptor antagonist with inverse agonist properties, decreased all the levels of cAMP observed in both cell lines. These results suggest that the agonist-independent (constitutive) activity of β₂-adrenoceptors plays a key role in the control of forskolin-induced cAMP accumulation.