Effects of testosterone (T) on upstream migratory behavior in masu salmon, Oncorhynchus masou, were studied by use of artificial raceways. In Experiment 1, yearling precocious males castrated in August were implanted with a capsule of medical silicone tube containing 500 μg of T in September. Their upstream migration was recorded in an artificial raceway with castrated and sham-operated precocious males implanted with a capsule containing vehicle for 2 months. In Experiment 2, upstream migratory behavior of yearling immature parr implanted with a capsule containing T 500 μg or vehicle was observed from September through November. In Experiment 3, upstream migratory behavior of castrated, castrated + T 50 μg, castrated + T 500 μg, and sham-operated precocious males was observed from September through October. In Experiment 4, upstream migratory behavior of the control, T 50 μg-, T 500 μg-, and T 1000 μg-treated immature parr was observed from September through October. In each experiment, plasma and pituitary samples were taken from the fish that moved upstream and remained to measure levels of T, gonadotropin (GTH) II, thyroxine (T4), and triiodothyronine (T3) by radioimmunoassay. Administration of T caused increases in plasma T levels within the physiological range. In Experiment 1, the frequency of migration upstream was 0, 19.2, and 35.7% in the castrated, castrated + T 500 μg, and sham-operated groups, respectively. The frequency was higher in the castrated + T 500 μg and sham-operated groups than in the castrated group. In Experiment 2, the frequency was higher in T 500 μg-treated parr (22%) than in the control (2.6%). In Experiments 1 and 2, pituitary contents of GTH II in the T-treated groups and precocious males were higher than those in the castrated precocious males and immature parr. In Experiment 3, castrated + T 50 μg, castrated + T 500 μg, and sham-operated fish showed upstream migratory behavior, whereas castrated fish without T did not. In Experiment 4, the frequency was 5.7, 22.9, 17.1, and 28.6% in the control, T 50 μg-, T 500 μg-, and T 1000 μg-treated groups, respectively. In each experiment, plasma levels of T4 in migrants were lower than those in nonmigrants, whereas plasma levels of T3 did not show such changes. From these results, it is inferred that T is a factor influencing upstream migration in masu salmon.