From incubation day 10 to 15, the spinal cord of Chick embryos are electrically stimulated in ovo at 0.5 Hz at the level of the motor roots innervating the latissimus dorsi muscles. As a consequence, the total number of spots of acetylcholinesterase determined on serial sections of posterior latissimus dorsi muscle increased 2.3 fold. This increase parallels that of acetylcholine receptor clusters, supporting the interpretation that these clusters are of synaptic origin.