The development and aging of four brain midline structures--the pituitary gland, pons, cerebellar vermis, and corpus callosum--were studied. The dimensions and area of these structures were measured by means of midsagittal magnetic resonance imaging. The study group consisted of 94 patients newborn to 15 years old and 49 patients and seven volunteers 16-60 years old. Except for growth spurts in the 1st year and in the 10-15-year age range, the pituitary gland showed linear growth. The pons, cerebellar vermis, and corpus callosum all showed exponential growth. The cerebellar vermis showed the sharpest 1st-year growth spurt, followed by the corpus callosum and the pons. The pituitary gland showed a decrease in size in the 51-60-year age range. The corpus callosum also showed a tendency to diminish in size but to a lesser degree. There were no statistically significant declines in the size of either the pons or the cerebellar vermis in the 51-60-year age range.