Gas transfer in fish gills is simulated by a simple counter-current model, with ventilation, water-blood transfer and blood flow characterized by conductances. The ventilation and perfusion conductances are products of flow rate and effective solubility. The diffusion conductance of water-blood transfer (diffusing capacity) is considered to depend on diffusion properties of both the water-blood tissue barrier and of interlamellar water. In the gills of the dogfish Scyliorhinus stellaris, more than half of the total resistance to O2 diffusion was located into interlamellar water. Complicating factors like water shunt, blood shunt, ventilation-perfusion maldistribution, pulsatile flow, diffusion in blood and reaction of O2 with hemoglobin may reduce the O2 transfer efficacy predicted by the simple model. In S. stellaris, the effect of such complicating factors appeared to be minor in most conditions, but in other species and/or conditions, more complex models might be required.