As juvenile salmon migrate from freshwater into saltwater, physiological changes must occur to allow fish to osmoregulate in the marine environment. Juvenile spring Chinook salmon migrate to the ocean at different time periods, most notably in the fall (August – November; i.e. fall smolts) and spring (February – May; i.e. spring smolts) after their first summer of rearing. We tested the ability of laboratory reared fall and spring smolts to osmoregulate when placed in salt water from freshwater by comparing differences in plasma sodium concentration after a 24-hr salt water challenge. The fish were reared under two different regimes in which we manipulated temperature and feed in order to produce different growth rates such that fall smolts exhibited a faster growth rate. We conducted four salt water challenges from 28 August – 11 December (approximately once per month). Interestingly, spring fish when tested in the fall still were able to be saltwater tolerant. All fish survived the salt water challenges. Fall smolts had significantly lower plasma sodium concentrations in the first two challenges, indicating a higher smolt quality. However, plasma sodium concentrations in fall smolts increased each month, suggesting they were reverting in smolt quality. Spring smolts had mean plasma sodium concentrations below the 170 mM/L threshold which has been suggestive smoltification. However, spring smolts had greater variation in plasma sodium concentrations with approximately 65% of the fish having levels below the smoltification threshold, compared to > 85% in fall smolts. The ability of fall and spring smolts to regulate plasma sodium during salt water challenges indicated both groups could survive in the ocean, suggesting that fall and spring smolts would migrate if there was a match between physiological and environmental factors.