The present study examined the temperature dependence of synaptic transmission and peptidergic modulation of chemical synapses on the phasic abdominal extensor muscles of crayfish. Decreasing the temperature from 25 degrees C to 5 degrees C in saline, decreased the EPSP amplitude by 88% and increased the EPSP half-decay time four-fold. The putative neurohormone DRNFLRFamide (DF2) increased EPSP amplitudes, but was more effective at 7-9 degrees C than at 15-17 degrees C. DF2 might play a hormonal role in counteracting low transmitter release at low temperature.