WE investigated the effects of heat shock on the temperature sensitivity of synaptic transmission in the motor circuit for flight inLocusta migratoria. In heat shocked animals synaptic transmission failed at 5–6°C higher than in control animals and recovery of transmission was more than three times faster upon return to room temperature. We also found that synaptic delay was rendered insensitive to increases in temperature by heat shock. Thus we have shown in the locust that heat shock has important protective effects on synaptic transmission, thereby extending the upper temperature limit for the motor patterns that generate flight. This is the first description of an effect of heat shock that preserves neuronal communication under subsequent stressful conditions.