The response of the endocrine system to short-term exposure to sea water (SW) was examined before, during and after the parr-smolt transformation of coho salmon. Five transfers of yearling coho salmon (Oncorhynchus kisutch) from fresh water (FW) to FW, or from FW to SW for 24 h were carried out between March and September. Condition factor declined significantly in late April, and gill Na⁺, K⁺-ATPase activity was elevated between late March and early June. Increases in plasma osmolality after transfer to SW were high in early March, low from late March — early June and slightly higher in September. In fish transferred from FW to FW, plasma levels of thyroxine (T₄) were elevated in late March-late April, and plasma levels of cortisol were high in late March-early June; both hormones declined to early March levels thereafter. Growth hormone (GH) levels increased in late March and remained elevated for the duration of the study. A significant linear correlation existed between plasma cortisol and growth hormone levels in FW to FW transferred fish. After SW transfer, T₄ levels were significantly lower than values in FW controls in early March and were significantly higher than FW control levels in early June and early September. Cortisol levels increased significantly after each transfer except in late April. However, compared with the early March and early September transfers, the magnitude of the elevation was reduced in late March. GH levels were significantly elevated above FW control levels after seawater transfer in late April only, despite a trend for mean GH levels to be higher after transfer in June and September. Thus, minimal increases in cortisol, and increases in GH occur after SW transfer of functional (optimally hypo-osmoregulating) smolts.