During childhood, serum melatonin concentrations drop by approximately 80%, but the 24-h melatonin excretion is stable. Arrest of pineal growth after the end of infancy has been proposed as one possible mechanism underlying that phenomenon. To test this hypothesis, we reviewed 332 magnetic resonance imaging brain studies, classified as normal, of endocrine-normal children, aged 1 day to 15 yr, and estimated the pineal and pituitary sizes. The pineal was identified in 277 of 332 magnetic resonance imaging studies (83%). The average size (mean +/- SEM) of the pineal gland (transaxial diameter, 5.6 +/- 2.1; midsagittal diameter, 5.0 +/- 2.4; planimetric area, 28.5 +/- 17.8) did not differ with age. A total of 74 of 277 pineals with cysts (26.7%) were found. The occurrence of pineal cysts was equally distributed among the different age groups (chi 2 = 11.6; df = 14; P = 0.7). Ten pineals showed more than 1 cyst (3.6%). The pituitary was identified in 325 of 332 brain images (97.9%). The average pituitary size increased by some 100% from 1 to 15 yr of age [transaxial diameter: F = 2.2; P = 0.005 (by two-way analysis of variance); midsagittal diameter: F = 3.7; P = 0.0001; planimetric area: F = 7.1; P = 0.0001]. The pituitary was slightly larger in females than in males [midsagittal diameter: F = 8.8; P = 0.003 (by two-way analysis of variance); planimetric area: F = 7.9, P = 0.005]. The data presented indicate a lack of a discernible pineal growth after age 1 yr, which contrasts with pituitary development in the same individuals. The data are in agreement with a hypothesis suggesting a growth arrest of the pineal after infancy.