1. Changes in partial pressures of O2 and CO2 in blood and water afferent and efferent to the gills are reported. These variables were measured before, during and after moderate swimming activity in rainbow trout.
2. Neither blood PCO2 nor water PO2, afferent or efferent to the gills, changed markedly before, during or after exercise.
3. Arterial blood was always more than 95% saturated with oxygen. Venous blood was 38% saturated, falling to a minimum of 28% during exercise.
4. PCO2 of arterial blood was 2.3 mm. Hg. Pco2 of venous blood increased from 5.7 to 8.0 mm. Hg during exercise and remained elevated throughout the recovery period.
5. Cardiac output (calculated using the Fick principle) stroke volume, ventilation volume and the volume of water pumped per breath all increase by a factor of between 4 and 5 during exercise. All tended to remain elevated for between 10 and 30 min. after exercise and then gradually decrease to pre-exercise levels.