Highly efficient oxygen uptake in elasmobranchs, as indicated by frequent excess of PaO2 over PEO2 has previously been ascribed to the operation of multicapillary rather than counter-current gas exchange by the gills. Analysis of models shows that, at maximum efficiency, a multicapillary system cannot account for values of PaO2 greater than (PIO2+PEO2)/2. In Port Jackson sharks Heterodontus portusjacksoni) PaO2 commonly exceeds (PIO2+PEO2)/2, which indicates the operation of a functional counter-current at the respiratory surface. The anatomical basis of this counter-current is provided by the demonstration that a continuous flow of water passes between the secondary lamellae into septal canals and thence via the parabranchial cavities to the exterior.