The ability of cortisol to increase gill Na⁺, K⁺-ATPase activity was examined in several salmonid species during development. Coho salmon (Oncorhynchus kisutch) parr were unresponsive to cortisol in vitro (10 μg/ml for 2 days) in November. Responsiveness was significant from January to March, peaking in January just prior to seasonal increases in gill Na⁺, K⁺-ATPase activity. Gill tissue became unresponsive to in vitro cortisol in April when in vivo gill Na⁺, K⁺-ATPase activity peaked. The ability of cortisol to stimulate gill, Na⁺, K⁺-ATPase activity in postemergent fry (2–3 months after hatching) was examined in chum (O. keta), chinook (O. tschawytscha), coho, and Atlantic salmon (Salmo salar). Initial levels of gill Na⁺, K⁺-ATPase activity were elevated in chum salmon, which normally migrate as fry. Cortisol (10 μg/ml for 4 days in vitro) increased gill Na⁺, K⁺-ATPase activity in chum salmon fry (48% above initial levels), had a limited but significant effect in chinook salmon fry, and had no effect in coho and Atlantic salmon fry. In an in vivo experiment, Atlantic salmon previously exposed to simulated natural photoperiod (SNP) and continuous light (L24) received four cortisol injections of 2 μg · g⁻¹ every third day. SNP fish responded with increased gill Na⁺, K⁺-ATPase activity (+66%), whereas L24 fish were not affected. Atlantic salmon presmolts with initially low levels of gill Na⁺, K⁺-ATPase activity responded to cortisol in vitro, whereas smolts with initially high levels of gill Na⁺, K⁺-ATPase activity were unresponsive. Triiodothyronine (0.01–10 μg/ml), prolactin (0.1–10 μg/ml), growth hormone (0.1–10 μg/ml), insulin (0.01–10 μg/ml), and bovine insulin-like growth factor I (0.01–1 μg/ml) did not affect gill Na⁺, K⁺-ATPase activity in vitro, individually or with cortisol (1–10 μg/ml). Thus, changes in responsiveness to cortisol occur during salmonid development, vary among species, and may be important in the heterochrony that characterizes the parr-smolt transformation.