1. A method of integrative flame photometry is described, by which the amount of sodium in biological fluids can be determined, using samples of the order of 10-3 cu.mm.
2. With pure solutions of sodium chloride, of concentration from 0.05 to 2.0 %, the average error of the method is about 3 % of the concentration.
3. When other substances are present systematic errors of up to 7.7 % have been recorded.
4. The possibilities of reducing these systematic errors and of extending the method to deal with other elements are discussed.