Experiments were performed on isolated detrusor smooth muscle from New Zealand White rabbits. Oxytocin was shown to exhibit high intrinsic contractile activity on isolated strips of detrusor muscle, where the maximum contractile amplitude was 12% greater than control responses to 1 μMcarbachol. Repeated applications of 1 μMoxytocin were associated with tachyphylaxis representing a 49% decrease in the amplitude which became reproducible after several applications without further decay of contractile strength. Dose–response experiments indicated that threshold contractions to oxytocin occur at 3 μMand were maximum at 10 μMwith mean effective concentration of 125 μM. The contractile responses to 1 μMoxytocin were not antagonized by phentolamine, atropine, methysergide, saralasin, or naloxone, but were partially inhibited by 1 μMof indomethacin. Ligand binding studies on partially purified membrane preparations from detrusor smooth muscle were performed over a range of 78 pMto 10 nMwith125I-labelled oxytocin. Scatchard analysis of specific bound receptors indicated aKDof 2.5 nMandBmaxof 187 fmol/mg protein and a second compartment that was unsaturable at the concentrations of ligand employed. Nonspecific binding ranged from 36 to 77% of the total binding.