Abstract 1. 1. The principal osmotic constituents of plasma and muscle have been determined. 2. 2. Compared with sea-water, plasma shows increases in sodium and calcium, with reductions in potassium, magnesium, chloride and sulphate. Plasma is isosmotic with sea-water. 3. 3. Ions form about 47% of the constituents analysed in muscle, amino acids 33%, trimethylamine oxide 6.9% and betaine 12%. 4. 4. Calculated osmolalities of sea-water and plasma using osmotic coefficients agree reasonably well with determined osmolalities.