1. The frequency (0.5-150 Hz) and calcium dependence (0.5-2.0 mM) of the effects of the nicotinic antagonist tubocurarine (0.2 microM) on acetylcholine (ACh) liberation from motor nerve terminals has been examined using binomial analysis of quantal transmitter release. 2. At an extracellular calcium ion concentration ([Ca2+]o) of 2.0 mM, tubocurarine produced a decrease in the endplate current (EPC) quantal content of approximately 30% at high frequencies of motor nerve stimulation (50-150 Hz). In contrast, at low frequencies of stimulation (0.5-1.0 Hz), tubocurarine enhanced the EPC quantal content by approximately 20%. 3. The enhancement of EPC quantal content produced by tubocurarine at low frequencies of motor nerve stimulation was [Ca2+]o dependent, being abolished when [Ca2+]o was lowered from 2.0 to 0.5 mM. In contrast, the decrease in quantal content produced by tubocurarine at high frequencies of motor nerve stimulation was independent of [Ca2+]o, being approximately 30% at all calcium ion concentrations studied. 4. In direct contrast to tubocurarine, the nicotinic antagonist vecuronium (1.0 microM) produced no increase in EPC quantal content at low frequencies of nerve stimulation. However, at high frequencies of nerve stimulation it decreased EPC quantal content to a similar extent to 0.2 microM tubocurarine. The frequency-dependent decrease in EPC quantal content produced by 1.0 microM vecuronium in 2.0 mM [Ca2+]o was very similar to that seen with 0.2 microM tubocurarine in 0.5 mM [Ca2+]o. 5. Binomial analysis revealed that all the changes in EPC quantal content associated with both nicotinic antagonists were due to changes in the size of the pool of quanta in the nerve terminal available for immediate release with no effect on the probability of release of an individual quantum. 6. The results are interpreted in terms of two separately identifiable prejunctional actions of the nicotinic antagonists, both involving an action at nicotinic ACh receptors situated on the motor nerve terminal. Thus, at high frequencies of motor nerve stimulation tubocurarine and vecuronium produce a [Ca2+]o-independent decrease in ACh release, probably through an inhibitory action on a positive-feedback prejunctional nicotinic autoreceptor closely related to the muscle-type nicotinic ACh autoreceptor. However, at low frequencies of motor nerve stimulation we suggest that tubocurarine, but not vecuronium, produces a [Ca2+]o-dependent increase in ACh release through an action at a negative-feedback prejunctional neuronal-type nicotinic ACh autoreceptor.