1. A study has been made of major aspects of the physiological adaptations for terrestrial life possessed by the amphibious clingfish, Sicyases sanguineus, on the coast of central Chile.
2. These fish can survive for more than 1½ days out of water, if not exposed to severe dehydration or thermal stresses. Rates of evaporative water loss while out of water are low. Upper lethal temperatures are also low, reflecting the uniformly cool water temperatures of their environment. In nature the fish rarely leave areas in which they are frequently washed over by waves.
3. These fish demonstrate an unusual combination of metabolic and cardiovascular adjustments to emersion. Metabolic rates (oxygen consumption) of small (1-5 g weight) fish are higher out of water than in water. Larger fish show a pattern of intermittent oxygen uptake when out of water. Heart rates respond to emersion (and associated cessation of breathing movements) in the pattern of the diving syndrome. Lactic acid concentrations in the blood gradually increase above control levels during the first few hours of emersion, then remain constant at about x 3 control level. There is no indication of peripheral vascular shutdown during emersion.
4. Emersion produces a marked shift towards ureotelism in waste nitrogen production. There appears to be no systemic accumulation of ammonia during emersion.
5. The generality of the results, and also their physiological significance, are discussed.