Endothelin-like immunoreactivity (ET-LI) was quantified in the developing (foetal and postnatal) rat brain stem and cerebellum using radioimmunoassay. The brain stem structures chosen for this study were (a) dorsal medullary region (DMR) including the region of nucleus tractus solitarius where the peripheral chemoreceptor afferents are known to terminate, (b) ventral medullary region (VMR) where the central chemoreceptors are thought to be located and (c) cerebellum (CER), as a control area. Compared to the prenatal period, significantly elevated concentrations of ET-LI were detected in the early postnatal period and thereafter the concentrations decreased: DMR and VMR: in comparison to the prenatal concentrations, a two-fold increase was found on the day of birth which further increased significantly (P < 0.001) on postnatal day 1 only in the region of DMR; CER: low concentrations of ET-LI were found in the early postnatal period which were not significantly different from the prenatal values. No ET-LI could be detected in any of the three regions in the adult rats. The results are discussed in view of the hypothesis that (1) endothelin appears to play an important role in the perinatal period and (2) it is involved in the chemoreceptor pathway.