Helcom?s recently revised Baltic Sea Action Plan (BSAP) aims to reduce eutrophication in the, Baltic Sea by reducing nutrient loads from all discharging drainage basins. The BALTCOST costminimisation, model, featuring abatement cost and effect functions which utilise spatial data down to, 10ױ0 km2 grid cell resolution, is used to identify a cost-effective distribution of nutrient abatement, measures between drainage basins whilst attempting to achieve specified nutrient load reductions for, separate Baltic Sea regions. Results indicate that the 2013 BSAP load reduction targets for nitrogen and phosphorus can be achieved in most Baltic Sea regions, except for phosphorus into the Baltic Proper, and the Gulf of Finland. The estimated minimum total cost of delivering these reductions is 4.17 billion, Euros annually, with substantial differences in the cost burden between countries and measures. Results are strongly influenced by differences in nutrient retention between drainage basins. Detailed, retention modelling and high spatial resolution source data are major novel features of this research.