The response of Retzius neurons, the main neuronal source of serotonin in the leech nervous system, to cholinergic agonists has been extensively investigated. In this study, we analyzed the effects of inhibiting the acetylcholinesterase (AChE) activity in the leech midbody ganglion on the electrophysiological activity of the Retzius neurons. Bath application of neostigmine and physostigmine (0.1-100 mumol l-1) produced, after a delay, a strong depolarization of the Retzius neurons with a dose-dependent amplitude and latency. The amplitude of this depolarization increased as the extracellular level of Ca2+ increased and decreased as the extracellular level of Ca2+ decreased. The response to neostigmine and physostigmine was inhibited by curare (100 mumol l-1), nicotine (10 mumol l-1), atropine (100 mumol l-1) and strychnine (100 mumol l-1), but was not affected by mecamylamine (100 mumol l-1) or hexamethonium (100 mumol l-1). Superfusion with solutions containing 100 mumol l-1 strychnine or atropine produced a progressive hyperpolarization of the Retzius neurons, while superfusion with 100 mumol l-1 curare did not. The hyperpolarization induced by atropine was inhibited in the presence of curare. Other neurons in the ganglion showed distinctive responses to the AChE inhibitors that were coincident with their responses to cholinergic agonists. The results suggest the existence of a basal level of acetylcholine (ACh) release in the leech ganglion that is powerfully counteracted by endogenous AChE activity. Under control conditions, this basal release appears to be sufficient to generate an ACh tonus that regulates the membrane potential of Retzius neurons. Since these neurons can support a sustained firing rate, which is dependent on the membrane potential, the results presented in this report suggest that the basal ACh tonus regulates the output of these neuromodulatory serotonergic neurons.