Evaluating the relative effects of local (e.g. eutrophication) and global (e.g. ocean acidification, OA) environmental change is important to predict how marine macroalgae might respond to future oceanic conditions. In this study, the effects of nitrate supply, and hence tissue nitrogen status, and OA on the N metabolism, growth and photosynthetic rates of the kelp Macrocystis pyrifera were examined. We hypothesized that (1) NO3- uptake and assimilation processes will depend on nitrate supply and (2) tissue N status modulates the physiological response of Macrocystis to OA. Macrocystis blades were grown for 3 days under replete or deplete NO3- concentrations. Thereafter, the NO3- replete and deplete blades were grown for 3 days under current and future pCO(2)/pH conditions, with NO3- enriched SW. After the initial pre-experimental incubation, total tissue N content, nitrate reductase (NR) activity and internal NO3- pools were reduced under low [NO3-], while NO3- uptake rates increased. Initial tissue N status did not modulate the physiological response to OA. However, NO3- uptake rates and NR activity were enhanced under the OA treatment regardless of the initial tissue N status, suggesting that increases in [H+]/reduced pH might play a regulating role in the N metabolism of this species.