The stretch of a frog muscle within the physiological range can more than double the spontaneous and evoked release of neurotransmitter from its motor nerve terminals. Here, stretch enhancement of release was suppressed by peptides containing the sequence arginine-glycine-aspartic acid (RGD), which blocks integrin binding. Integrin antibodies also inhibited the enhancement obtained by stretching. Stretch enhancement depended on intraterminal calcium derived both from external calcium and from internal stores. Muscle stretch thus might enhance the release of neurotransmitters either by elevating internal calcium concentrations or by increasing the sensitivity of transmitter release to calcium in the nerve terminal.