Common carp juveniles were fed two types of diet (control: 0.1% tryptophan and TRP: 0.6% tryptophan) over 15 days. Thereafter, both groups were directly subjected to osmotic challenge (from 0 to 10 ppt) for 168 h. Blood samples were collected at -240, 0, 6, 24, 72 and 168 h after challenge. Survival and serum cortisol, glucose, sodium and chloride levels were measured to determine stress response and osmoregulation condition. While TRP group showed no mortality until 168 h, cumulative mortality was near 100% at 72 h after challenge in control group. Feeding tryptophan-supplemented diet led to increase in prechallenge cortisol, but not glucose, sodium and chloride values at -240 h that stayed elevated until 0 h after challenge. After challenge, cortisol and glucose values did not significantly affected by time of sampling but type of diet and diet×time interaction. Sodium values significantly affected by diet type and time of sampling but not their interaction. Chloride values significantly affected by time of sampling but not diet type and their interaction. Control group had higher cortisol, glucose and sodium than TRP at each time of sampling. Control and TRP group showed increasing and decreasing pattern in cortisol and glucose values when experiment progressed after osmotic challenge. Both group showed increasing pattern in sodium and chloride values when experiment progressed after osmotic challenge. Results indicated that tryptophan supplementation enhanced salt water tolerance of carp that is due to increase in basal cortisol and anti-stress effect of tryptophan and possibly increase in serotonergic activity.