thors outline the underpinning theory of short-term exchange rate determination in the framework of the portfolio balance approach. Optimal policy under flexible exchange rate regimes is illustrated, under the assumption that monetary authorities act to minimise the cost of divergence from target values of the exchange rate money supply and reserve stock. Three cases can be distinguished in the implementation of their system of equations: no intervention, intervention with incomplete sterilisation, and intervention and sterilisation. The developed approach is then applied to the data of US dollar - Deutsche mark exchange rate for the period