Underyearling fall chinook salmon (Oncorhynchus tshawytscha) were reared at two temperatures (9 and 15°C) and three photoperiods simulating winter (WIN), spring-summer (SS), and natural (NAT) conditions. The two artificial photoperiods were adjusted according to fish size so that an equinoctial photoperiod was synchronized with a hypothesized optimum size range for smolting (8-14 cm fork length). Fish exposed to a winter (WIN) photoperiod had the slowest growth rate. Fish under the SS photoperiod at both temperatures and under the WIN photoperiod at 9°C displayed hypoosmoregulatory capability near the equinoctial daylength. Thyroxine concentrations in fish reared under a natural photoperiod remained at basal levels during the period of early saltwater adaptiveness and throughout the remainder of the experiment. Thyroxine titers showed significant increases under all experimental photoperiod-temperature combinations near the equinoctial daylength with the exception of the 15°C/SS treatment.