Three- or four-year-old Atlantic salmon (Salmo salar) which first smolted in their second spring (S2 smolts) and had been kept in freshwater since hatching have the external characteristics of parr in December and smolts in April. Such fish were used to investigate the acute effects (up to 48 h) of rapid changeover from freshwater to seawater on respiratory, acid-base and ionoregulatory homeostasis. Over the first 18 h of seawater exposure the plasma sodium, chloride and total osmotic concentrations rose rapidly in a similar manner for both groups of fish. In the smolt-like fish levels then stabilised whereas in parr-like fish they continued to rise to the end of the experiment. Immediately after exposure to seawater, there was a rapid rise in blood pH and a fall in arterial Pa O2 which were transient in the smolt-like and more severe and prolonged in the parr-like fish. These results are discussed in the context of branchial dehydration and cell volume control. It is suggested that an important aspect of smolting is the ability to regulate branchial dehydration since this must precede activation of the branchial ion excretory mechanisms regulating extracellular fluid composition following seawater transfer. © 1989.