Abstract The present paper describes the results of a study on the prenatal development of muscle activity in the sheep. Recordings have been made of the mechanical and electrical responses of the gastrocnemius muscle in fetuses (weight 0.24–1930 gm) in placental contact with the ewe. In the earliest part of the active period of the fetus the duration of the mechanical response of the gastrocnemius muscle is about 0.8 to 1 sec and the latency of the response about 6 msec. With increasing gestational age, contraction time and relaxation time become shorter and the latency of the response decreases. About the one-hundredth day of the gestational period the twitch takes less than 0.3 sec and the latency of the response has decreased to 1.5 or 2 msec. Parallel with these changes there is an increase in frequency of the stimulation required to obtain fusion of contraction. As development proceeds, the conduction velocity of the nerves rises from values less than 1 m per sec during the early stage of the active period to 35 to 75 m per sec after the one-hundredth day. This change coincides with the myelination of the nerve fibers.