1. Some fresh-run lamperns can osmoregulate in 50% sea water (Δ = 0.970° C), where they can maintain plasma freezing-point depressions of about 057° C.
2. An analysis of the mechanism of osmoregulation in these animals shows that it is similar in many respects to that employed by marine teleosts. The lampern swallows sea water and absorbs a solution containing a high proportion of monovalent ions into its blood. It has been calculated that chloride is excreted by an extra-renal route, presumably by means of chloride excretory cells which have been discovered in the gills. The rate of extra-renal loss of water is high and the urine output is negligible.
3. Many fresh-run animals are unable to osmoregulate. In some cases the capacity to swallow sea water becomes reduced, whilst in others the number of chloride excretory cells is diminished--an indication that the extra-renal excretory mechanism is failing. Both of these mechanisms regress simultaneously in some animals.