Summary.The osmotic pressure and the salinity of the blood ofCallorhynchus antarcticusand ofEpiceratodus forsterihave been investigated with a view to determining the relations between internal fluids and the aquatic environment in the Holocephali, and the Dipnoi.Live fish were examined, and the specimens were quite normal (theCallorhynchuswere examined at Hobart, Tasmania, and theNeoceratodusat Brisbane, Queensland).The blood of theCallorhynchushad a freezing‐point almost the same as the sea water in which the specimens were living. The blood contained over 2·6 per cent. urea and chlorine representing 1·33 per cent. NaCl.These figures indicate thatCallorhynchuspresents the extraordinary conditions found in only one group of the animal kingdom—the Elasmobranchii.The blood ofEpiceratodushas a relatively high freezing‐point, —0·42°C., compared with —0·5 to 0·6 for freshwater Teleostei, and —·56 to —0·6 for mammalia. There is no resemblance whatever to Elasmobranch conditions. The Dipnoi fall into the same category as the higher vertebrates. The freezing‐point of the blood is curiously li