Daily variations in melatonin binding sites in the brain of underyearling masu salmon Oncorhynchus masou were examined by radioreceptor assay using 2-[ 1 2 5 I]iodomelatonin as the radioligand. Fish were reared under a natural photoperiod in July and sampled eight times at 3 h intervals from 12:00 to 09:00 h. Plasma melatonin levels showed robust daily rhythms in both precocious males and immature females, with high and low levels during night and day, respectively. The affinity (K d ) and density (B m a x ) of melatonin binding sites in the brain also showed similar variations. There were significant positive correlations between the plasma melatonin levels and the K d or the B m a x in immature females and between the K d and B m a x values in both precocious males and immature females. These results indicate that melatonin binding sites in the brain showed daily variations under a natural photoperiod in masu salmon.