Albino Sprague-Dawley rats (1, 8, 12 and 20 days old) were injected intra- peritoneally with l-[^3H-l]-methadone (5 mg/kg containing 50 μCi/kg), and sacrificed at 15 min, 1 h and 3 h. Whole brain was homogenized in 0.32 M sucrose-Tris buffer and the homogenates were subjected to differential centrifugation to separate nuclei, mitochondria, microsomes, soluble cytosol, myelin, membrane and synaptosomes. Methadone levels in each fraction were examined. The methadone contents in the whole brain (sum of all fractions) of 1- and 8-day-old rats were significantly higher relative to those of 12- and 20-day-old rats at all time intervals. The most striking finding in this study was that during development, the percentage of methadone content in the synaptosomal fraction progressively increased relative to other subcellular fractions; interestingly, there was an associated decrease in the percent levels in the soluble cytosol suggesting a shift of methadone from cytosol to synaptosomes. These alterations in brain synaptosomal accumulation of methadone could result from the progressive increase in the affinity of methadone for synaptosomes as these particles become enriched in the protein and lipid contents and possibly increase in opiate receptor density during the neuronal maturation and the synap- togenesis of the developing brain.