1. At the crustacean neuromuscular junction, action potential-evoked neurosecretion increases in proportion to stimulation frequency, a process termed frequency facilitation. In the present study we examined how frequency facilitation is affected by osmotic pressure. 2. Hypertonic solution (HS) was applied by local superfusion of the synaptic area. Quantal release was monitored by focal extracellular recordings of postsynaptic potentials. Several stimulation frequencies (f) in the range from 1 to 10 Hz were employed, and quantal content (m) together with the number of releasable units (n) and release probability (p) was evaluated for each frequency. 3. Osmotic pressure enhanced quantal release at the lowest f tested (1 Hz) but suppressed neurosecretion at higher f (7-10 Hz). Thus, hyperosmolarity enhanced action potential-evoked release but suppressed frequency facilitation. 4. Chelation of intracellular calcium by BAPTA showed that the effect of HS was calcium independent. 5. Binomial analysis of quantal content revealed that HS suppressed the increase in the number of releasable units, which was very pronounced during facilitation under control conditions. Since HS also stimulated asynchronous quantal release, the observed effect of HS on facilitation can be explained by the depletion of the releasable pool of quanta caused by the asynchronous neurosecretion. 6. To test this hypothesis we increased the available pool of vesicles using serotonin and demonstrated that the suppressing effect of HS on facilitation was reversed. 7. The observed effects of HS on facilitated neurosecretion could be described quantitatively using our model for mobilization of vesicles into the releasable pool enhanced by action potentials.