The cerebral distribution and regulation of excitatory amino acid levels may play a crucial role in neuronal development. In the present study we examined concentrations of the endogenous excitatory amino acid antagonist kynurenic acid and related substances during development in fetal and neonatal rat brain and fetal non-human primate cerebral cortex. Kynurenic acid concentrations in rat fetal whole brain were significantly increased 4-5 fold prenatally, then declined rapidly at 1 day after birth, and reached adult concentrations at 7 days after birth. L-Kynurenine concentrations were also markedly increased prior to birth and then declined to adult concentrations at 1 day after birth. L-Tryptophan was increased 3 fold before birth, and decreased to adult concentrations 1 day after birth. In contrast concentrations of dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid and homovanillic acid increased 1 day prior to birth and continued to increase following birth. Fetal baboon cerebral cortex showed significant increases in kynurenic acid concentrations both pre-term and near-term as compared with adult concentrations. These results show that marked changes in kynurenic acid concentrations occur prior to and following birth. It is possible that high levels of kynurenic acid prior to birth inhibit neurite branching and development of excitatory synapses, which then develop rapidly in parallel with the decrease in kynurenic acid levels.