Activation of spontaneous motility produced by the systemic administration of strychnine (1 mg/kg egg weight) and picrotoxin (1 mg/kg egg weight) was studied in 13- and 17-day-old chick embryos. In both cases the effect was studied in normal embryos, in embryos decapitated 1, 24 and 48 hours previously and in embryos decapitated on the 2nd day of incubation at stage 11-13 according to Hamburger and Hamilton (1951). At 13 days, both substances had the same short-term activating effect on the controls and all the decapitated embryos. This result is evaluated as further evidence of non-participation of the supraspinal component in the spontaneous motility of chick embryos before the 15th day of incubation. In 17-day-old decapitated embryos, strychnine and picrotoxin activation of spontaneous motility was significantly smaller than in the controls. From a comparison of activation by strychnine and picrotoxin in normal and chronically decapitated embryos it was deduced that a spinal and a supraspinal component participated in their effect. The activating effect of strychnine and picrotoxin in 17-day-old embryos was found to be mediated largely by supraspinal factors.