MicroRNAs (miRNAs) play a broad range of roles in biological regulation. In this study, rat pineal miRNAs were profiled for the first time, and their importance was evaluated by focusing on the main function of the pineal gland, melatonin synthesis. Massively parallel sequencing and related methods revealed the miRNA population is dominated by a small group of miRNAs as follows: ~75% is accounted for by 15 miRNAs; miR-182 represents 28%. In addition to miR-182, miR-183 and miR-96 are also highly enriched in the pineal gland, a distinctive pattern also found in the retina. This effort also identified previously unrecognized miRNAs and other small noncoding RNAs. Pineal miRNAs do not exhibit a marked night/day difference in abundance with few exceptions (e.g. 2-fold night/day differences in the abundance of miR-96 and miR-182); this contrasts sharply with the dynamic 24-h pattern that characterizes the pineal transcriptome. During development, the abundance of most pineal gland-enriched miRNAs increases; however, there is a marked decrease in at least one, miR-483. miR-483 is a likely regulator of melatonin synthesis, based on the following. It inhibits melatonin synthesis by pinealocytes in culture; it acts via predicted binding sites in the 3"-UTR of arylalkylamine N-acetyltransferase (Aanat) mRNA, the penultimate enzyme in melatonin synthesis, and it exhibits a developmental profile opposite to that of Aanat transcripts. Additionally, a miR-483 targeted antagonist increased melatonin synthesis in neonatal pinealocytes. These observations support the hypothesis that miR-483 suppresses Aanat mRNA levels during development and that the developmental decrease in miR-483 abundance promotes melatonin synthesis.