1. The influence of sex and body size on the osmotic pressure, chloride and free amino acids in the blood of the freshwater field crab, Paratelphusa sp., and the freshwater mussel, Lamellidens marginalis, were investigated.
2. In the crab the osmotic pressure increased in both the sexes to a maximum at about 40 g. in males and at 35 g. in females and then fell with further increase in weight. Throughout the whole size range the males tended to have a higher osmotic pressure.
3. Blood chloride in the crab also increased with weight in both the sexes to a maximum at about 40 g. in males and 35 g. in females and then steadily decreased as the weight increased. In the positive slope of the regression line the females have a higher blood chloride, but in the negative slope of the curve the males have higher blood chloride.
4. The free amino acid content of the blood reached a maximum at about 32 g. in males and 35 g. in females and then gradually decreased as the weight increased. Over the whole size range the males tended to have higher free amino acid content than the females.
5. In the freshwater mussel both chloride and free amino acid content of the blood showed a small but steady increase with increasing weight. This is also reflected in a significant increase in the blood osmotic pressure with increase in body weight.