The effects of EtOH on the crayfish Procambarus clarkii and P. simulans were examined behaviorally in vivo and electrophysiologically in vitro on pre- and postjunctional mechanisms of synaptic plasticity at opener excitor nerve-muscle junctions. Addition of 75 mM EtOH to the bath water of holding tanks produced 47 to 54 mM EtOH levels in the hemolymph (blood) within 24 hr. These hemolymph EtOH levels were maintained for weeks by daily changes of the bath water containing 75 mM EtOH. After 24 hr of exposure to 75 to 150 mM EtOH in vivo, crayfish showed behavioral signs of intoxication as measured by a significant increase in righting reflex times and a significant decrease in tail-flip escape behavior. After 2 weeks of chronic exposure to 75 mM EtOH, crayfish showed behavioral tolerance as measured by a decrease in righting time and an increase in tail-flip escape behavior to control levels. EtOH applied acutely to opener nerve-muscle preparations in vitro at 10 to 100 mM concentrations produced an increased probability of transmitter release as measured by an increased frequency of spontaneous release of transmitter quanta and an increased amplitude of facilitated synaptic potentials evoked by 10 to 40 Hz stimulation of the excitor axon. Acute application of 300 to 600 mM EtOH resulted in a decreased amplitude of facilitated synaptic potentials due primarily to a decrease in postsynaptic input resistance. These data suggest that EtOH has a concentration-dependent biphasic effect on synaptic transmission.