This study examines the effects of inhibition of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) on acetylcholine (ACh)-induced contraction in rat urinary bladder smooth muscle. Neostigmine, a non-selective ChE inhibitor, caused concentration-dependent contractions in rat urinary bladder strips, whereas tetraisopropylpyrophosphoramide (iso-OMPA; a BuChE inhibitor) failed to affect the resting tone of the preparations. Neostigmine (1M) markedly augmented the contractile responses to ACh. Although iso-OMPA (10M) also potentiated ACh-induced contraction, the effect was less than that evoked by neostigmine. The activities of AChE in rat urinary bladder strips were significantly (P0.05) higher than those of BuChE. These results indicated that AChE, rather than BuChE, plays an important role in controlling ACh-induced contractions of rat urinary bladder.