The continuous administration of d-tubocurarine (6.5 ± 0.4 mg/kg e.w./24 h) to chick embryos from the 4th to the 12th day of incubation had a positive effect on defects produced in the development of spontaneous motility either by decentralization of the spinal cord or by chemical phénobarbital depression, or by a combination of both experimental factors. In normal embryos, d-tubocurarine had no effect on the development of spontaneous motility.