In goldfish serum, cortisol was found to constitute 77.6% of the adrenocorticosteroids measured by a competitive protein-binding radioassay. Adult goldfish maintained on a photoperiod of 14 h light: 10 h dark in November exhibited no significant variation in serum corticosteroid concentration throughout the 24-h cycle. Goldfish maintained in an 8L:16D photoperiod in June exhibited two peaks in serum adrenocorticosteroid concentration. Four hours before the onset of the light period and 4 h after the onset of the light period, serum corticosteroids were significantly higher than those observed at the midpoint of the dark period. After sham injection, swimming in shallow water, or a thermal shock, but not a handling disturbance, circulating levels of corticosteroids were significantly higher than in undisturbed fish. Betamethasone injected 24 h before a thermal stress completely blocked the stress-induced increase in serum corticosteroids observed in vehicle-injected and uninjected goldfish, demonstrating the potency of this steroid as a blocker of the pituitary–interrenal axis in this species.