We describe a novel preparation of the isolated brain stem and spinal cord from prometamorphic tadpole stages of the South African clawed frog ( Xenopus laevis) that permits whole cell patch-clamp recordings from neurons in the ventral spinal cord. Previous research on earlier stages of the same species has provided one of the most detailed understandings of the design and operation of a central pattern generator circuit. Here we have addressed how development sculpts complexity from this more basic circuit. The preparation generates bouts of fictive swimming activity either spontaneously or in response to electrical stimulation of the optic tectum, allowing an investigation into how the neuronal properties, activity patterns, and neuromodulation of locomotor rhythm generation change during development. We describe an increased repertoire of cellular responses compared with younger larval stages and investigate the cellular-level effects of nitrergic neuromodulation as well as the development of a sodium pump-mediated ultraslow afterhyperpolarization (usAHP) in these free-swimming larval animals. NEW & NOTEWORTHY A novel in vitro brain stem-spinal cord preparation is described that enables whole cell patch-clamp recordings from spinal neurons in prometamorphic Xenopus tadpoles. Compared with the well-characterized earlier stages of development, spinal neurons display a wider range of firing properties during swimming and have developed novel cellular properties. This preparation now makes it feasible to investigate in detail spinal central pattern generator maturation during the dramatic switch between undulatory and limb-based locomotion strategies during amphibian metamorphosis.