During the spring smoltification of the coho salmon (Oncorhynchus kisutch), alterations in osmoregulatory function occur in the urinary bladder, intestine and opercular membrane. Na and Cl reabsorption by the urinary bladder of seawater-adapted fish, initially at freshwater levels, is abolished during the period of smoltification. Intestinal fluid absorption of freshwater-adapted fish increases to seawater levels. The abundance of mitochondria-rich "chloride cells" in freshwater-adapted fish opercular membranes increases. All of these changes occur some weeks after the plasma thyroxine surge and coincide with increased brancial Na,K-ATPase activity. These data suggest that the appropriate time for seawater entry or placement of hatchery-reared coho salmon may be several weeks after the new moon-related thyroxine peak.