Magnetic resonance imaging (MRI) of the mesencephalon during the first four years of life allowed normal maturational processes of the various midbrain structures in vivo to be followed. Using T2-weighted SE sequences, we found 5 characteristic age dependent patterns on axial tomograms taken at the level of the superior colliculi, that let us derive a grading system for normal development of the quadrigeminal plate, the cerebral peduncles, the reticular substantia nigra and the red nuclei. A subsequent statistical analysis of these age dependent changing patterns on T2-weighted MRI of 60 neonates, infants and small children yielded normal age ranges for each of the 5 maturational stages of the midbrain. Grading the changing pattern of midbrain structures during early postnatal life into 5 distinct maturational stages allowed not only monitoring of normal differentiation, e.g. myelination of the brainstem in vivo, but may also help to distinguish between normal, delayed and abnormal development of the mesencephalon on routine MRI.