Postsynaptic activity may modulate presynaptic functions by transsynaptic retrograde signals, At developing neuromuscular synapses in Xenopus nerve-muscle cultures, a brief increase in the cytosolic calcium ion (Ca2+) concentration in postsynaptic myocytes induced persistent depression of presynaptic transmitter secretion, This depression spread to distant synapses formed by the same neuron, Clearance of extracellular fluid did not prevent the spread of depression, and depression could not be induced by increasing the Ca2+ concentration in a nearby myocyte not in contact with the presynaptic neuron, Thus, the spread of depression is mediated by signaling in the presynaptic cytoplasm, rather than by a retrograde factor in the extracellular space.