We examined the distribution of carbonic anhydrase in dorsal root ganglia and spinal cords of chick embryos from Stage 26 (5 days) through hatching using the Hansson technique (Hansson, H.P., Histochemistry, 11 (1967) 112-128; Lannerholm, G., Ann. N.Y. Acad. Sci., 429 (1984) 369-381). Although reactivity was apparent in this tissue at all stages (e.g. in endothelial cells, erythrocytes, and cell nuclei) carbonic anhydrase-positive dorsal root ganglion neurons were not detected until Stage 35 (8.5-9 days). At this stage fewer than 1% of neurons were reactive. The number and proportion of carbonic anhydrase-positive dorsal root ganglion neurons increased steadily from Stage 35 to hatching, when about 60% of the neurons were reactive. In hatchlings carbonic anhydrase-positive neurons spanned the entire size spectrum of ganglion cells and appeared to be randomly distributed throughout the dorsal root ganglion. No carbonic anhydrase-positive neurons were found in the spinal cord at any stage. However, in hatchlings the background staining was markedly lighter in laminae 1 and 2 and in Lissauer's tract than in the rest of the cord, suggesting that unmyelinated axons in the CNS are not reactive.