Benzodiazepine receptor binding and the concentration of the free tissue GABA, glutamate, glutamine, aspartate, and 13 other amino acids were estimated in specimens of cerebral cortex and cerebellum taken from normal foetuses and from perinatal and postnatal infants and adults postmortem. Compared with the ontogenetic increase, previously reported, in [3H]muscimol binding, that of [3H]flunitrazepam binding developed more rapidly in the cerebellum and less rapidly in the cerebral cortex. The concentration of GABA in both brain parts increased with age more rapidly than did that of glutamate decarboxylase activity. The overall pattern of developmental changes in the amino acid concentrations in the human brain showed many similarities with those reported in the brains of experimental animals. The concentration of the amino acids associated with the tricarboxylic acid cycle increased with age. Although the general downward trend in the levels of essential amino acids (with the exception of Met) was also evident in the human cerebral cortex, this was not the case in the cerebellum where the concentrations of most amino acids were relatively low in the foetus whereas the concentrations exceeded the cerebral cortical levels in the adult.