Abstract The effect of a range of concentrations of nitrate (NO − 3) on the growth rate and nitrate reductase (NR) activity of both young and mature sporophytes of Laminaria digitata (Huds.) Lamour has been studied by means of laboratory batch culture experiments. The growth rate of young sporophytes was found to increase in a hyperbolic fashion with increasing NO − 3 availability, with a k s value of 19 μmol·dm −3. The potential in vivo NR activity of these plants (obtained under optimum assay conditions) remained constant over the range of NO − 3 concentrations used, while the actual in vivo NR activity (sustained by the internal NO − 3 pool within the cell) increased in a similar hyperbolic manner to that shown by the growth rate ( k s 20 μmol·dm −3). The changes in the actual in vivo NR activity were consistent with those of the internal NO − 3 content of these plants, which also increased with increasing external NO − 3 concentration. The NR activity in the blade meristem of the mature sporophytes behaved in a similar manner to that of the entire young plants. In contrast, the potential in vivo NR activity of the old, non-meristematic region of the blades of mature plants (where the maximum NR activities were located) did respond to the external availability of NO − 3, being greater in those plants grown in high concentrations of NO − 3 than in those in which growth was nitrogen-limited. In addition to this trend, a similar dependence of the ratio of actual : potential NR activity on the degree of nitrogen limitation to that found in the young sporophytes occurred in this region of the blade of mature plants. Pronounced diurnal variations in NR activity, with maximum values in the light period and minimum in the dark, were observed in both field and laboratory populations of L. digitata. The amplitude of these fluctuations appeared to be controlled by the degree of nitrogen limitation experienced, being much greater when growth was light- rather than nitrogen-limited (minimum values 44 and 74% of maximum, respectively). Overall the data indicate that the ratio between the actual : potential in vivo NR activity in L. digitata provides an unambiguous indicator of the state of the nitrogen metabolism within the cells, the interpretation of which, unlike growth rate, is not affected by differences in other culture or environmental conditions. This finding is believed to have important implications for the commercial cultivation of this and other species of macroalgae.