An extensive survey of the whole of the North Sea, carried out in July 1987, is described. The study concentrated on the measurement of surface water inorganic nutrient concentrations and concomitant rates of primary production (14C) and nitrogen assimilation (15N). Primary production was investigated using size fractionation techniques. Three vertical profiles of primary production and nitrogen assimilation were also investigated. Much of the North Sea exhibed thermal stratification. Surface nutrient concentrations were low and chlorophyll concentrations typically <1 mg m-3. More than 75% of the primary production was attributable to cells <5 μm in diameter. Ammonium assimilation accounted for most of the nitrogen assimilation. The water column was vertically well mixed in the coastal zones. Here, inorganic nitrogen concentrations were high (e.g. up to 25 μmol NO3- with chlorophyll concentrations up to 10 mg-3, and organisms >5 μm diameter accounted for most of the primary production. As in offshore regions ammonium accounted for the major part of the nitrogen assimilated. A 115 km section obtained using an undulating oceanographic recorder showed that in certain regions of the North Sea physical features acted to increase the dependence of the phytoplankton on nitrate.