The changes in distribution and number of GABA immunoreactive (GABA-ir) fibers from postembryonic stages to adulthood in the lamprey spinal cord white matter were studied by using immunocytochemical techniques. From prolarvae to adult spawning animals there was an increase of the number of GABA-ir fibers. Three phases can be distinguished: (a) from prolarvae to middle size larvae (around 50 mm in body length) an increase in the number of GABA-ir fibers per section is observed. Furthermore, an adult-like pattern of GABA-ir fibers distribution is established during this phase. (b) Then, the number of GABA-ir fibers remains stable until metamorphosis, the end of the larval period. (c) Finally, in young postmetamorphic and adult animals the number of GABA-ir fibers is higher than in larvae. These observations, joined to the changes previously reported in the GABA-ir neurons, indicate that at least parts of the GABA inhibitory component of the spinal locomotor network is reorganized during the lamprey life cycle and it may indicate different inhibitory requirements in the locomotor network.