Following the administration of the monoamine oxidase inhibitor, pargyline, turnover rates of the biogenic amines norepinephrine (NE), dopamine (DA) and serotonin (5HT) were determined in discrete brainstem nuclei: substantia nigra (SN-A9), dorsal raphe (dr), nucleus gigantocellularis (rgi), locus coeruleus (LC-A6), nucleus ambiguus (n amb) and nucleus tractus solitarius (nts) of young and adult rabbits. The initial concentration of the amines varied markedly among the various nuclei studied, with higher values predominating in the older animals. The turnover rates for NE and DA in the rgi and LC were lower in young animals. The turnover rates for DA in the dr and nts were greater in the young animals. In addition, young rabbits had higher turnover rates for 5HT in the nts. The average turnover times for NE and 5HT were longer for the young animals (17.5 h vs 4.1 h) and (8.3 h vs 3.5 h) respectively. These data point to maturational differences in neurochemistry which may aid in elucidating possible mechanisms of respiratory instability in newborns.