Adult sockeye salmon (Oncorhynchus nerka) in Lake Shikotsu were captured in September, October and November adjacent to their natal hatchery prior to spawning. They were sampled for hormones, tagged and released in the center of lake. Fish were again sampled at recapture to characterize changes in steroid hormone levels in individual migrants as well as homing percentage and duration in each month. All males returned faster than females early in the breeding season, although a half of the tagged males did not return to the natal site late in the season (November). A high percentage of females always returned, and homing duration shortened late in the season. In males, the shortening of homing duration coincided with an increase in serum testosterone (T) and 11-ketotestosterone levels. In females, the shortening of homing duration corresponded to an elevation of serum T and 17α,20β-dihydroxy-4-pregnen-3-one (DHP) levels, and a drop in serum estradiol-17β levels. Sustained administration of gonadotropin-releasing hormone analog (GnRHa, via implants) in September greatly shortened homing duration, especially in females. GnRHa treatment caused a dramatic increase in serum DHP levels in both sexes on average. Individual GnRHa-treated males which rapidly returned, however, showed higher serum T levels and lower serum DHP levels than slower returning males. The present study indicates sexual differences in homing profiles as well as shortening of homing duration following GnRHa implantation in lacustrine sockeye salmon in Lake Shikotsu which may be reflective of changes in serum steroid hormone levels.