Solifenacin is an antimuscarinic agent used to treat symptoms of overactive bladder. Pharmacologically significant amounts of solifenacin were excreted in the urine of humans taking a clinical dose of this drug. The aim of this study is to measure muscarinic receptor binding in the bladder urothelium and detrusor muscles of rats following the intravesical instillation of solifenacin. Muscarinic receptors were measured by radioreceptor assay using [N-methyl-³H]scopolamine methyl chloride ([³H]NMS), a selective radioligand of muscarinic receptors. Solifenacin showed concentration-dependent inhibition of specific [³H]NMS binding in the bladder urothelium and detrusor muscle of rats, with no significant difference in K_i values or Hill coefficients between these tissues. Following the intravesical instillation of solifenacin, there was significant muscarinic receptor binding (increase in K_d for specific [³H]NMS binding) in the bladder urothelium and detrusor muscle of rats. Similar bladder muscarinic receptor binding was observed by the intravesical instillation of oxybutynin, but not with trospium. In conclusion, the present study has demonstrated that solifenacin binds muscarinic receptors not only in the detrusor muscle but also in the bladder urothelium with high affinity. These bladder muscarinic receptors may be significantly affected by solifenacin excreted in the urine.