In the marine green alga Ulva rigida C. Agardh, nitrate reductase (NR) is synergetically induced by blue light and nitrate. The present study examines the effect of blue light and a large NO 3 pulse (0.3 raM) on relevant variables of NO3-assimilation such as NO3-uptake, intracellular NO3-storage, NR activity, in vivo NO;--reduction rate and NO 2 and NH~--accumulation. Nitrate uptake started immediately upon addition of NO3, suggesting the presence of a constitutive carrier, however in the first 1.5 to 2 h, periods of net NO 3 effiux were frequent. After this time, NO3-uptake and intracellular NO3-accumulation proceeded linearly with time, suggesting the existence of a different NO3-uptake mechanism, which seems to be inducible. Our results indicate that in vivo NO3-reduction is not exclusively dependent on the potential NR activity. In U. rigida, during the first 2 h after a NO 3 pulse (300 ~tM) there were clear indications that the induction state of the N03-carrier limits the reduction rate of NO3. Once the induction of the N03-transporter had been completed (1.5 to 2 h), the NO3-assimilation pathway reached a steady state, N0y-uptake rate, NO3-reduction rate and N02 and NH~-accumulation being linear with time. Since the reduction of NOy leads mainly to the accumulation of NHJ, we conclude that, after the NO~--reduction itself, NH~--fixation into carbon skeletons is the limiting step in the assimilation of NO~-by U. rigida under blue light. Communicated by J. M. Pdr~s, Marseille