Nitrogen uptake by net- (15-200 mum), nano- (1-15 mum) and picoplankton (<1 mum) was measured over seasonal cycles at two stations with different patterns of biological and chemical cycles in the Morlaix Bay (western English Channel). Though assimilable dissolved N nutrient pool at both stations was nitrate-dominated, characteristics of biomass and N uptake by netplankton differed from conventional patterns in two respects. In the first, biomass (26-30%) and N uptake (36-43%) were less important than those of nanoplankton. In the second, the netplankton did not show any marked preference for nitrate over ammonium (nitrate to ammonium uptake ratios of 0.98 and 1.08). In contrast, nanoplankton had a preference for ammonium over nitrate (ammonium to nitrate uptake ratios of 2 and 1.2). N uptake by picoplankton was only 8% of total N uptake at both stations and was supported mainly by regenerated N (66% ammonium and 17% urea), with nitrate uptake detectable in only one instance and nitrite uptake in none. Substrate-dependent uptake of ammonium in all fractions and a higher ammonium uptake in the nanoplankton fraction in summer at both stations when ambient ammonium concentrations were high indicated that while nitrate may satisfy a part of N requirements, availability of ammonium and its flux through nanoplankton determine the magnitude of total N uptake in these waters. Most of the N uptake in picoplankton appears to be autotrophic, suggesting that a substantial part of heterotrophic uptake, if any, could be localized in the fractions >1 mum, and mediated by free-living and particle-bound bacteria.