The effect of bcta-carboline (Ji-CCE) on spontaneous motility and its development was studied in chick embryos between the 11th and 19th day of incubation. 1. Acutely administered p-CCE (7.5 mg/kg e.w.) already induced significant activation of motility in 11-day-old embryos. From the 17th day of incubation activation acquired a paroxysmal character. 2. In spinal embryos (decapitated on the second day of incubation) there was no such activating effect, demonstrating that it is associated with supraspinal components of the CNS. 3. In chronic administration from the fourth day of incubation (1.55 ± 0.24 mg/kg e.w./24 h), P-CCE led to reduced development of spontaneous motility. The effect was concentrated in the period between the fourth and eighth day of incubation. The chronic administration of P-CCE augmented the activating effect of metrazol and weakened GABA-inhibition of spontaneous motility. 4. On the basis of their findings, the authors express the hypothesis that the benzodiazepine /1-CCE-scnsitive component of the complex GABA receptor evidently already functions from the beginning of the second half of incubation of chick embryos.