The networks of neurones in the spinal cord which control vertebrate locomotion, commonly called central pattern generators or CPGs, are constructed very early in development, often before locomotion is even possible. These elementary networks are then modified during development to suit changing behavioural requirements. The CPGs must therefore serve the needs of an organism at its particular stage in development and at the same time lay the foundations for the control of movements at later stages. How do the CPGs controlling locomotor behaviour change during maturation to cope with this challenge? This article reviews recent data from several preparations, but especially the developing swimming system of amphibian embryos, in which the mechanisms underlying. ÇPG development are beginning to unfold. In Nenopus, the descending projections of brainstem neurones which release 5HT appear to be instrumental in the postembryonic development of the spinal CPG, via actions on a range of cellular and synaptic mechanisms.