Steady state levels of gamma-hydroxybutyrate (GHB) were measured in whole brain and discrete regions of brain in developing and adult rat, monkey, and human brain. Postmortem changes in concentration of GHB in rat and human brain were also assessed. There were no significant postmortem changes in GHB under the conditions which the ontogeny experiments were done. The concentration of GHB was uniformly higher in the immature brains of the 3 species studied. In the rat the highest concentration was in immature hypothalamus and cortex with a significant decrease occurring between postnatal day 12 and 14. In human, the highest concentration was in fetal cerebellum and adult hypothalamus. Comparison of these data with published ontogeny data for gamma-aminobutyric acid (GABA) suggest that there may be a source of GHB in brain other than GABA.