Synaptic transmission of the single gene mutant, shibirets1 (shi), of Drosophila melanogaster is reversibly blocked by elevated temperature. The presynaptic mechanism of transmission was studied in the neuromuscular junction of the dorsal longitudinal flight muscle of this mutant. It was observed that when the temperature was raised to 29 degrees C in shi flies, the amplitude of the excitatory junction potential (EJP) greatly diminished, the frequency of spontaneously released miniature excitatory junction potentials (MEJP's) was greatly reduced, and almost complete vesicle depletion was observed. These conditions were reversible if the temperature was lowered to 19 degrees C. These data suggest that the block in transmission is a result of vesicle depletion. It is suggested that depletion occurs not as a result of excessive release of transmitter but rather as a result of a block in the recycling of vesicles, which causes depletion as exocytosis (transmitter release) proceeds normally.