Abstract
An analysis is given which makes it possible to trace out the changes in oxygen tensions in the blood and water during their passage along a secondary lamella of the dogfish gill. The analysis depends on a knowledge of the oxygen-dissociation curve of the blood, the shape of the secondary lamella and the oxygen tensions of the two media before and after their passage through the gills. It indicates the differences to be expected according to whether the flows are co-current or counter-current. The method, with modifications, could be applied to the gills of all fishes.