The development of the motor reaction to i.v. injection of the sodium salt of penicillin G in a dose of 0.9 x 10(6) I.U./kg egg weight was studied in chick embryos (normal and spinal) from the 11th to the 19th day of incubation. Penicillin first caused standard activation of embryonal motility from the 15th day of incubation, in both normal and spinal embryos. Activation was at first continuous in character (a twofold increase in the frequency of spontaneous movements). In 17- and particularly in 19-day embryos a typical paroxysmal reaction developed, with pronounced intervals of motor rest. The proportion of the spinal component in the penicillin reaction was abut 40% and of the supraspinal component about 60% of total motor activity. In older embryos (after the 15th day of incubation), the motor reaction to penicillin could be effectively modified by the systemic administration of glycine and GABA. It is concluded from the results that penicillin does not activate embryonal motor activity until a given stage of development of the CNS has been attained. It is a developmental phenomenon with a spinal and a supraspinal component, in which central inhibitory mechanisms participate.