The motor output of the leg of the chick embryo during hatching (20 days of incubation) was characterized using electromyographic (EMG) recordings from identified knee and ankle muscles. A highly coordinated pattern of motor output was found. It was therefore used as a standard against which to compare the motor output from younger embryos (7, 9, 13, 17 and 19 days of incubation). Despite large differences in some aspects of the EMG records from embryos of different ages, consistent patterns of muscle activation were observed. Quantitative analysis of phase and latency relationships between pairs of muscles indicated that at least some elements of the neural circuitry involved in generating the hatching motor output may be laid down very early in development. Duration vs. latency plots revealed that there is a gradual refinement in the temporal pattern of alternation between antagonist muscles during development.