Thirty-four females and 27 males of a species of rockfish, Sebastiscus marmoratus were kept in tanks circulating sea water from March 15 to 28, 1968. They were acclimatized to three series of osmotic pressure changes. The changes of the osmotic pressure of serum and serum protein fractions derived from electrophoresis were compared for each series.
1. A close linear relation was observed, with the correlation coefficient of +0.84, between the osmotic pressure of sea water and its chlorinity. No correlation was observed between the osmotic pressure of rockfish serum and that of the sea water of the experimental tank.
2. A positive correlation was observed between the specific gravity of blood and haematocrit. However, no correlation were obtained between the specific gravity and the amount of serum protein, and also between haematocrit and the amount of serum protein.
3. Each electrophoretic fraction of serum protein, I, II, III and IV plus V, in specimens kept in natural sea water with about 1040 mOsm/kg showed almost no difference as compared with those of the normal pattern. However, component IV plus V increased with the lowering and raising of the osmotic pressures of sea water, whereas components I, II and III scarecely varied.