During the R/V Wecoma WEC88 cruise that sampled a meridianal transect along 150¿W from 15¿N to 15¿S, uptake of nitrate and ammonium by phytoplankton was measured using the stable isotope 15N with simulated in situ bottle incubation at shipboard mass spectrometry. A set of 25 daily productivity stations showed the influence of equatorial upwelling on nitrate distribution and 15N uptake in a band from 6¿N to 7.5¿S compared with the oligotrophic waters to the north and south, with the highest values of nitrate uptake occurring at the equator (0.14 mg-at. m-2 h-1). During a 5-day time series at the equator, there was an increase in nitrate accompanied by increased nitrate uptake. Interestingly, nitrate uptake rates (equivalent to new production rates) at the equator were lower than those predicted by previous investigators. Holdover experiments and uptake versus irradiance curves showed that the phytoplankton there were in an early stage of metabolic adaptation and that may have been a contributing factor. ¿ American Geophysical Union 1991