The report describes the activities and results from 1988 within the open sea programme of the national Swedish monitoring programme , PMK . The open sea programme also forms the major part of the Swedish contribution to the Baltic Monitoring Programme of the Helsinki Commission. Three regular cruises were carried out in the Kattegat, five in the Sound, two in the Baltic and four in the Gulf af Bothnia. Standard oceanographic parameters including nutrients were anlysed and various biological samples were taken including fish and mussel samples fer analysis af harmful substances. Resulting from a warm winter, rich in precipitation, the sea water remained comparatively warm in most areas and in the Skagerrak even until late in the autumn. Also in 1988 the south-eastern part af the Kattegat had a period af very low oxygen concentrations in the bottom water in September-October. The area concerned was larger than before. Although a small inflow to the Baltic occurred in September nothing has changed in the oxygen conditions af the deep waters in general. In the Bornholm Deep hydrogen sulphide was present during the greater part of the year and during the summer it also was present in the Hanö Bight. In the deep water of the Gotland Deep the hydrogen sulphide has been present for 11 years and the concentrations are higher than ever. No hydrogen sulphide was present in the Landsort Deep, but the oxygen concentrations were very low. Petroleum hydrocarbons were present in low concentrations in the water, but with no difference from previous vears.