Seasonal and spatial changes in the rate of phytoplanktonic primary production were measured in the eutrophic Colne Estuary and the factors, such as dissolved inorganic nutrients, light and biomass which regulated it were examined using experimental and statistical techniques. The aim was to determine the factor(s) most affecting primary production which is important in estuarine management. Seasonal and spatial variation in the rate of primary production was significant (p 0.05) with higher values over summer (max. 17 microgram C l-1 h-1). The chl a values were higher over summer (37.4 microgram l-1). Light had an important effect on the rate of primary prodcution and biomass measured as chl a, shown by multiple regression models over winter. All around the year nutrient concentrations decreased down the estuary. During the summer season phytoplankton biomass and production was largely affected by the dissolved inorganic nutrients. Nutrient addition experiments clearly showed stimulation by increased ammonium of both production and biomass at both ends of the estuary in July, 1996. The depletion experiments gave clear evidence of faster utilisation of ammonium than the rest of the nutrients, while silicate was the only nutrient which was depleted by the end of the experiments between May and November, 1996. This showed that the diatom fraction of the phytoplankton population was potentially silicate limited.The N-15 uptake experiments confirmed that ammonium was preferred to nitrate by the phytoplankton. In summer (May and June) phytoplankton assimilated > 95 % of nitrogen as ammonium, though in September 99 % (percent) of nitrogen assimilation was from nitrate in the oligohaline part of the estuary. In the euhaline part of the estuary in June and September, 1996 >75 % (percent)of nitrogen assimilation was in the form of ammonium despite much lower concentrations of ammonium than nitrate.The average depth and time integrated priamry production was calculated to be 34 microgram C m-2 y-1 with a modeling method that does not take tidal effect into account. This figure was not high compared to other hypernutrified estuarine systems but higher than in the open sea. The phytoplanktonic nitrogen uptake was > 2 % (percentage) of total nitrogen input to the estuary.