A comparison has been made of the patterns of muscle activity during swimming in the intact and spinal lamprey, and the patterns of ventral root activity in the in vitro preparation of the lamprey spinal cord. Electromyographic (e.m.g.) activity was recorded with intramuscular bipolar electrodes from three segmental levels in intact lampreys swimming in a swim-mill at a range of swimming speeds. The patterns of activity obtained were similar to those seen in elasmobranch and teleost fish. After high spinal transection, lampreys could be induced to swim continuously for a period of several minutes in the swim-mill by a light initial mechanical stimulation of the tail or dorsal fin. The patterns of e.m.g. activity obtained from spinal animals at a range of swimming speeds were similar to those obtained in the intact state. Portions of spinal cord were isolated encompassing those segments from which e.m.g. recordings had been made and ventral root recordings were made in vitro of the rhythmic activity induced by bath application of D-glutamate. In all experiments the mean duration of the bursts of activity at any segmental level was directly proportional to the mean cycle duration, and the constant of proportionality (about 0.36) was similar for all three types of preparation. In all preparations the mean time delay for the activation of segments in the rostral-caudal direction was proportional to the cycle duration and to the number of segments between recording positions. The proportionality constant, or phase lag per segment, was approximately equal to 0.01 in all three types of preparation.