Changes in β-adrenergic receptor density under long-term application of ritodrine were investigated using longitudinal muscle strips of day-21-pregnant-rat myometrium. Membrane (––)[3H]dihydroalprenolol binding sites were increased by 3.1 × 10––8 Mritodrine, with a maximum increase at 3.1 × 10––7M, followed by a gradual decrease at higher concentrations. Under long-term application of a lower dose (1.5 × 10––7M) of ritodrine, the number of the binding sites increased up to 20 min, then decreased to the control level and fluctuated after 90 min. These results suggest that at lower doses, ritodrine might activate adenylate cyclase, resulting in conformational changes in the receptors, and that receptor density might vary in parallel with cellular cAMP production under long-term application of ritodrine.