The effects of ovine growth hormone (oGH) and cortisol on downstream migratory behavior in yearling (1 +) smolts and underyearling (0 +) parr of masu salmon, Oncorhynchus masou, were examined during the downstream migratory period in spring using artificial raceways. In May, each of 22 1 + smolts and 0 + parr were implanted with cholesterol pellets containing 250 μg of oGH and/or 2 mg of cortisol. Their downstream migratory behavior was subsequently observed in artificial raceways, along with control groups 4–23 days after implantation. In 1 + smolts, the frequency of downstream migratory behavior was 23%, 18%, 72%, and 82% in the control, oGH, cortisol, and oGH + cortisol-treated groups, respectively. The frequency was significantly higher in the cortisol and oGH + cortisol-treated groups than in the control and oGH-treated groups. In 0 + parr, the frequency of downstream migratory behavior in the cortisol (82%) and cortisol + oGH-treated (90%) groups was significantly higher than in the control (18%) and oGH-treated (0%) groups. These results indicate that cortisol is an important endocrine factor inducing downstream migratory behavior in both 1 + smolt and 0 + parr of masu salmon.