Purpose: To correlate pineal parenchyma volume (PP) to circadian melatonin profiles and to determine the 24-hour melatonin per volume of pineal tissue (MLPV). Furthermore, we compared melatonin profiles of cystic and solid glands. Methods:In 15 healthy male subjects (20 -27 years) the PP was determined using high-resolution magnetic resonance imaging (trueFISP, isotropic voxel: 0.8 mm 3 ). Melatonin plasma concentrations (MC) were determined every 2 hours for 24 hours.Results: Mean PP was 125 Ϯ 54 mm 3 . PP correlated linearly to maximum MC and to 24-hour melatonin (r ϭ 0.61 and r ϭ 0.64, P Ͻ 0.05) but not to minimum MC. The 24-hour melatonin was 653 Ϯ 242.6 pg/mL, MLPV was 5.8 Ϯ 2.3 pg/mL*ml Ϫ1 . Nine pineal glands were solid and six were cystic. PP of solid glands showed higher linear correlations to maximum melatonin, to 24-hour melatonin, and to the slope of the melatonin increase of the curve (P Ͻ 0.05). Conclusion:PP correlates linearly to melatonin plasma concentrations in humans. MLPV displays similar interindividual differences as 24-hour melatonin. PP of solid glands show better correlation to melatonin profiles than cystic.