Changes in the levels of the brain catecholamines, norepinephrine (NE) and dopamine (DA), were studied in the male Japanese quail during maturation. In addition the neurotransmitter, serotonin (5HT), its precursor, tryptophan ( TRYP ), and its metabolite, 5-hydroxyindoleacetic acid (5HIAA), were measured. Birds were sampled at weekly intervals between the ages of 1 and 63 days of age. Brains of the quail were dissected into cerebral hemispheres, diencephalon, brain stem, cerebellum, and optic lobes. Spectrophotofluorometric analyses of the monoamines in the brain parts were performed following the validation of this technique for use with the avian brain. Norepinephrine increased significantly (P less than .05) with age in all brain regions except the cerebellum, whereas dopamine increased significantly (P less than .05) only in the cerebral hemispheres. Serotonin levels increased significantly (P less than .05) in all brain regions except the cerebellum. The most dramatic change over maturation was observed in the cerebral hemispheres, and relatively smaller increases occurred in the diencephalon. Tryptophan decreased significantly (P less than .05) in concentration in all brain regions throughout the 9 weeks of development. However, 5HIAA did not change significantly in brain regions. Serum testosterone, as measured by testosterone radioimmunoassay, increased in conjunction with sexual maturation. In general, the monoamine levels rose with age. Additionally, the monoamine distribution appeared to increase in caudal to rostral direction. Changes in the monoamines, particularly in the diencephalon, occurred at approximately the time that increased peripheral concentrations of testosterone were also observed. This observation supports the contention that monoamines may be involved in regulating gonadotropin release during maturation.