1. A method of flame photometry is described, by which the amount of sodium and potassium in biological fluids can be determined simultaneously, using samples of the order of 10 -3 cu.mm. 2. When the method is tested with quantities near the lower limits of detection (of the order 4 x 10 -8 mg. sodium, 6 x 10 -8 mg. potassium) the reproducibility, measured as standard deviation, is ±7 x 10 -9 mg. sodium and ± 17 x 10 -9 mg. potassium. 3. When larger quantities (up to certain limits) are used, the standard deviation is approximately ± 3 % of the quantity in the sample, both for sodium and for potassium. 4. As is usual in flame photometry serious errors can be caused by the presence of other substances in the sample. In the case studied for purposes of illustration it is shown that these interference errors can be reduced to the order of ± 6 % by the addition of excess ammonium phosphate.