The activation patterns in the longissimus and gastrocnemius muscles during the development of the mature type of locomotion were studied in rats from the 11th day until maturity. The EMG's of these muscles were recorded and movements were simultaneously recorded on videotape. Results indicate that during locomotion at early ages, the longissimus muscles are activated irregularly. From the 16th day, the bursts in the longissimus muscle become more pronounced. During locomotion, they are activated differentially and often a simultaneous activation of the longissimus and the contralateral gastrocnemius muscle was observed. From the 20th day, bursts in the gastrocnemius muscle during walking coincide with bursts in the ipsilateral longissimus muscle. Our results demonstrate that the mature type of postural control (from the 20th day) develops a few days after the development of the mature type of fluent walking (around the 15th-16th day).