Whales, as pelagic marine mammals, are thought to have evolved from fresh-water-dependent terrestrial mammals. Baleen whales feed primarily on salty euphausiids (krill) and have no access to fresh water. How have these mammals adapted to lifelong habitation in a hyperosmotic medium? A new approach is proposed for studying this by using allometry (scaling) of endogenous creatinine clearance in mammals together with determinations of creatinine concentration in fresh postmortem blood and urine of fin whales (Balaenoptera physalus) and sei whales (Balaenoptera borealis). From the predicted mean creatinine-clearance values and the measured mean creatinine concentrations, a urine-production rates of 974 and 627 L/day for the fin and sei whales, respectively, were computed. Average daily krill ingestion of about 1300 and 835 L is predicted for the fin and sei whales, respectively. The whales seem to ingest about 30% more than earlier reported of a prey, which has about 50% of the salt concentration of seawater, thus maintaining the salt and water balance with a minimum of 12% seawater ingestion. The method used to estimate the above volumes could be a valuable tool in further studies of the water and salt balance of the large baleen whales, which may not have the same osmoregulatory control mechanisms as the smaller Odontoceti.