The development of gamma-aminobutyric acid (GABA)-immunoreactive neurons was investigated in the embryonic and posthatch chick lumbosacral spinal cord by using pre- and postembedding immunostaining with an anti-GABA antiserum. The first GABA-immunoreactive cells were detected in the ventral one-half of the spinal cord dorsal to the lateral motor column at E4. GABAergic neurons in this location sharply increased in number and, with the exception of the lateral motor column, appeared throughout the entire extent of the ventral one-half of the spinal gray matter by E6. Thereafter, GABA-immunoreactive neurons extended from ventral to dorsal regions. Stained perikarya first appeared at E8 and then progressively accumulated in the dorsal horn, while immunoreactive neurons gradually declined in the ventral horn. The general pattern of GABA immunoreactivity characteristic of mature animals had been achieved by E12 and was only slightly altered afterwards. In the dorsal horn, most of the stained neurons were observed in laminae I-III, both at the upper (LS 1-3) and at the lower (LS 5-7) segments of the lumbosacral spinal cord. In the ventral horn, the upper and lower lumbosacral segments showed marked differences in the distribution of stained perikarya. GABAergic neurons were scattered in a relatively large region dorsomedial to the lateral motor column at the level of the upper lumbosacral segments, whereas they were confined to the dorsalmost region of lamina VII at the lower segments. The early expression of GABA immunoreactivity may indicate a trophic and synaptogenetic role for GABA in early phases of spinal cord development.(ABSTRACT TRUNCATED AT 250 WORDS)