Regional weight, protein concentration, and monoamine concentration were determined in the pons/medulla, neocortex, hippocampus, and cerebellum of 21- and 42-day-old offspring of rats exposed to either air or 75, 150, or 300 ppm carbon monoxide (CO) throughout pregnancy. Norepinephrine and serotonin concentrations decreased linearly with increasing CO exposure concentrations in the pons/medulla of CO-exposed offspring at 21 days of age, but not by 42 days of age. Norepinephrine concentration increased linearly with increasing CO exposure concentrations in neocortex and tended to increase in hippocampus of CO-exposed offspring at 42 days of age, but not at 21 days of age. Regional weights and protein concentrations of the neocortex, hippocampus, or pons/medulla were not affected by CO exposure at either age. Cerebellar weights of CO-exposed offspring, however, decreased linearly with increasing CO exposure concentrations at both 21 and 42 days after birth. No significant effect of CO exposure on cerebellar monoamine concentrations was observed. The results suggest that prenatal CO exposure disrupts the development of noradrenergic and serotonergic neuronal systems, and the development of the cerebellum.