Recent studies of the spinal motor systems of vertebrates allow us to begin to infer the organization of the motor apparatus of primitive vertebrates. This paper attempts to define some of the features of the motor system of early vertebrates based on studies of the motor systems in anamniotes and in Branchiostoma. It also deals with some changes in the primitive motor system during evolution. The primitive motor system consisted of myomeric axial muscles, with a functional subdivision of the musculature into non-spiking slow muscle fibers segregated in the myomeres from spiking fast ones. These fibers were innervated by two major classes of motoneurons in the cord - large motoneurons innervating faster fibers and small motoneurons innervating slow fibers. There was not a simple isomorphic mapping of the position of motoneurons in the motor column onto the location of the muscle fibers they innervated in the myomeres. Early vertebrates used these axial muscles to bend the body, and the different types of muscle fibers and motoneurons reflect the ability to produce slow swimming movements as well as very rapid bending associated with fast swimming or escapes, The premotor network producing bending was most likely a circuit composed of a class of descending interneurons (Dls) that provided excitation of ipsilateral motoneurons and other interneurons, and inhibitory commissural interneurons (CIs) that blocked contralateral activity and played an important role in generating the rhythmic alternating bending during swimming. This DI/CI network was retained in living anamniotes. At least two major descending systems linked the sensory systems in the head to these premotor networks in the spinal cord. The ability to turn on swimming by activation of DI/CI premotor networks in the cord resided at least in part in a midbrain locomotor region (MLR) that influenced spinal networks via projections to the reticular formation. Reticulospinal neurons were important not only for initiation of rhythmic swimming but also in the production of turning movements. The reticulospinal cells involved in turns produced their effects in part via monosynaptic connections with motor neurons and premotor interneurons, including some involved in rhythmic swimming. A prominent and powerful Mauthner cell was most likely present and important for rapid escape or startle movements. Some features of this primitive motor apparatus were conserved during the evolution of vertebrate motor systems, and others changed substantially. Many features of the early motor system were retained in living anamniotes; major changes occur among amniotes. Some of these changes include the breakup of the myomeres into a large number of discrete axial muscles, as well as the development of paired fins and limbs and the associated limb muscles. The primitive segregation of fiber types was lost in many of the muscles arising from the myomeres. Non-spiking slow fibers and spiking fast ones were retained in most vertebrates but were lost in most mammalian muscles. The motor column innervating axial muscles changed substantially with the development of a topographic map of the motor column onto the myotome in the embryo. The map probably arose at or before the evolution of amniotic vertebrates. The predominant mode of activation of axial muscles also changed within amniotes from the primitive 'swimming' motor pattern with alternation of activity on opposite sides and a rostrocaudal wave of muscle activity to a bilateral activation of muscles in mammals. However, even some amniotes, particularly reptiles, sometimes use motor patterns with primitive features. The change in motor pattern in mammals points to possible changes in the central circuits controlling the motoneurons. The major features of reticulospinal pathways are conservative among vertebrates. One of the most striking conservative pathways arises from the midbrain locomotor region that activates spinal networks for rhythmic movements in both fishes and mammals, even though there are substantial differences in the muscles, the movements and, possibly, the central pattern generators producing the movements in these animals.