ABSTRACT: A dynamic model has been developed describing the effects of transient N assimilation following NH₄⁺ pulses on protein synthesis and on C mobilization in red algae. The model simulations indicate that the differential response ofphycobiliproteins to N availability seems to be related to a more general response of chloroplast proteins to N supply. The model displays a high robustness. The incorporation of different functions of amino acid transport between the chloroplast andcytosol fractions, as well as different initial distributions of amino acids between these fractions, has little effect on N incorporation at the protein level, with chloroplast proteins being much more affected than cytosolic ones by the variation of theexternal forcing function, the NH₄⁺ supply. With respect to cell C metabolism, the main changes promoted by a transient NH₄⁺ assimilation were not in total cell C but in the allocation of C between C reservestructures (carbohydrates) and organic N compounds (amino acids and proteins). The stoichiometry of 6 C molecules needed per N molecule assimilated seems to be crucial in determining the rate of C mobilization in response to transient N assimilation. Thedevelopment of the model provides further insights in the mechanism of C-N interaction in marine red algae, where the presence of particular N compounds such as phycobiliproteins and of C compounds such as cell wall polysaccharides and floridean starch isdifferent compared to green algae and higher plants. The results of the simulations compared favorably with the experimental data reported for the red alga Gracilariopsis lemaneiformis.