A method was developed for the automatic quantitative determination of potassium in serum, by flow injection analysis. The serum sample is directly introduced in the flow. The dilution with deionized water stream is done automatically and sequentially in the system to reach the adequate concentration to be introduced in the flame photometer. The results achieved with this method, for serum samples obtained in the Hospital of the University, had been compared with those values of the Hospital laboratory that uses manual dilution and flame photometry. The results obtained by the two methods agree within at least ±5%. About 60 samples/hour can be easily analyzed by the method proposed in this work.