The concentrations of the biogenic amines, norepinephrine, dopamine and serotonin, were determined in 5 respiratory-related brainstem regions by use of a micropunch technique and a sensitive enzymatic isotopic assay. Samples were taken from rabbits at different ages (term fetuses, 3, 7, 14, 21 days and 2-year-old adults). All brain regions contained norepinephrine, dopamine, and serotonin in measurable amounts, but the distribution was not uniform. Norepinephrine and dopamine remained relatively low (less than 5 ng/mg) in the fetus and during the first weeks of life. Norepinephrine achieved its highest levels in the nucleus tractus solitarius and the locus coeruleus of adult animals. Dopamine concentrations did not change significantly in any nuclear groups over the ages tested. Compared with the newborn values, high concentrations of serotonin were seen in all brainstem nuclei in the term fetus. Adult serotonin levels were not achieved during the neonatal period. These data demonstrate pre- and postnatal changes in neurotransmitter levels in brainstem nuclei which regulate respiratory control. The susceptibility of the newborn to abnormalities in respiratory control may be related to low serotonin levels observed during the postnatal period.