In studies on 11 rainbow trout (Salmo gairdneri), 217.3 ± 6.5 g, 14.0 ± 0.5 C) fitted with rubber membranes for direct measurement of ventilatory water flow, buccal, cleithral, and differential pressure traces increased in amptitude asrose from 40 to 160 ml/min. Ventilation rate remained steady at around 77 breaths/min in therange 40–120 ml/min and rose to 90 breaths/min at aof 175 ml/min. Oxygen uptake more than doubled asrose from 40 to 120 ml/min. Calculated utilization of oxygen from the inspired water declined only slightly over thisrange, indicating that large increases in nonrespiratory spillage of water (anatomical dead space) had not occurred. The calculated oxygen cost of breathing was low. Area mean differential pressure appeared closely related toover therange 40–160 ml/min. Area mean differential pressure increased from two- to four fold over thisrange, depending upon which individual fish was examined. Over therange 40–160 ml/min the calculated resistance of the gill sieve did not appear to decline although some variability was present and a variety of gill sieve resistances could be measured at a givenfor each fish. It is concluded that changes inare largely dependent on changes in mean differential pressures across the gills, with no marked decline in gill sieve resistance over the normal physiological range of.