The routine oxygen consumption by Periophthalmus cantonensis and Boleophthalmus chinensis in water increased geometrically, whereas that in air increased logarithmically with temperature. At temperatures of more than 20 degrees C the oxygen uptake of both species was greater in water than in air. 2. When the fishes were able freely to select either an aquatic or terrestrial habitat, the total oxygen consumption of Periophthalmus and Boleophthalmus was 236 and 110 ml/kg, h at 20 degrees C respectively; 66% (Periophthalmus) and 70% (Boleophthalmus) of the total uptake was from water, and 34 and 30% of the total uptake was from air at 20 +/- 1 degrees C. 3. Oxygen uptake of fish limited to aquatic or terrestial life was less than when they could freely select their habitat; for Periophthalmus, uptake was reduced to 83% when confined in water and to 50% in air, and for Bolephthalmus, to 65% in water and to 43% in air. 4. The proportion of oxygen uptake by the gill in water was 52% for Periophthalmus and 59% for Boleophthalmus; in air the corresponding figures were 27 and 52%. 5. The proportions of oxygen uptake via the skin in water was 48% for Periophthalmus and 36% for Boleophthalmus; in air the corresponding figures were 76 and 43%. 6. It is concluded that, on land, Periophthalmus relies mainly on its skin and Boleophthalmus relies mainly on its gills.
