The total cortisol content in tilapia was 55 pg immediately following fertilization, then decreased abruptly and maintained a lower level of 10-20 pg until hatching; after hatching the cortisol content increased to 50 pg by the 7th day. Fertilized eggs were incubated in either 32% saltwater or fresh water and sampled at various developmental stages. Both groups showed dramatic changes in cortisol content following development. However, no significant difference in the cortisol level was found between the two groups. Tilapia larvae, hatched in fresh water, were reared with feed containing 0 (control) or 150 mg/kg wt/day cortisol, corticosterone, 17 alpha-hydroxyprogesterone (17 alpha-OHP) or 11 alpha-deoxycortisol (11 alpha-DC) for 8 days and then transferred directly to 27.5% saltwater. Those reared with corticosterone, 17 alpha-OHP, or 11 alpha-DC, similar to the control, all died within 4-8 hr after the transfer. However, the larvae treated with cortisol showed a much higher survival rate of 40-60%. The tissue osmolality in the control larvae, 394.3 +/- 3.7 (mmol/kg), increased abruptly after transfer to 26% saltwater and reach a peak, 681.5 +/- 47.5, before the larvae all died (12th hr after the transfer). In contrast, tissue osmolality in cortisol-treated (150 mg/kg wt/day for 12 days) larvae was 570.7 +/- 62.6 at the 12th hr and then began to decrease to 448.5 +/- 9.4 at the 24th hr and 386.0 at the 48th hr. These findings suggest that cortisol could play a critical role in the hypoosmoregulation in tilapia larvae.