The ontogeny of the interrenal stress response in rainbow trout was characterized by measuring resting and acute-stress-induced changes in whole-body cortisol levels in embryos and larvae at different early developmental stages. In Experiment 1, resting cortisol levels averaged 6.0 ng/g in newly fertilized eggs, fell to less than 0.3 ng/g by the time of hatching at Week 4 (incubation at 10 degrees), and increased to 1.4 ng/g by Week 5. Cortisol levels did not change in response to acute stress in 3-, 4-, or 5-week-old fish. In Experiment 2, resting cortisol averaged 1.4 ng/g in newly fertilized eggs, fell to less than 0.03 ng/g by Week 2, and then steadily increased between Weeks 3 and 6 to a peak of 4.8 ng/g before falling to 1.2 ng/g by Week 7. Cortisol levels did not change in response to acute stress in 3-, 4-, or 5-week-old fish. Six-week-old fish showed a 2.3-fold increase in cortisol levels at 1 hr poststress, indicating that the hypothalamic-pituitary-interrenal axis first develops responsiveness to stress 2 weeks after hatching and 1 week before the onset of exogenous feeding. The stress hyporesponsive period after hatching in rainbow trout may be homologous to the 2-week stress hyporesponsive period after birth in rodents, the function of which may be to maintain low, constant corticosteroid levels during a critical developmental period when these steroids can have permanent effects on neural organization. As suggested for mammals, this period may be a time when rainbow trout are particularly vulnerable to environmental effects on their subsequent development.