Abiotic stressors can affect organism performance through perturbed aerobic respiration, and are revealed by changes in oxygen consumption and by−products of the catabolism of respiratory substrates. When protein is used as a respiratory substrate, increased rates of nitrogenous excretion can result, and in aquatic invertebrates this corresponds to ammonium production. Elevated environmental pCO2 (hypercapnia) has been shown to affect aerobic respiration and nitrogen excretion in terrestrial and marine invertebrates, and this effect has been hypothesized to reflect the effects of pCO2 on acid−base regulation and protein metabolism. Thus, in the case of ocean acidification (OA) negatively affecting scleractinian corals, we reasoned it was timely to ask whether these effects might reflect changes in respiration and protein metabolism. To test the hypothesis that OA affects the respiration and excretion rates of juvenile Seriatopora caliendrum, corals were exposed for 14 d to 47 (ambient) versus 90 Pa (elevated) pCO2 at 27.5°C and assessed for total protein content, O2 consumption, NH4+ excretion, and the density of the coral s algal symbiont Symbiodinium spp. To block NH4+ recycling by Symbiodinium, the photosynthetic inhibitor DCMU was used in a contrast with uninhibited corals. Corals at 90 Pa pCO2 exhibited elevated rates of NH4+ excretion, whereas corals at 47 Pa continued to absorb NH4 + from seawater. However, pCO2 had no effect on respiration, protein content, or Symbiodinium density, and DCMU had no effects on any dependent variables. Our preliminary results suggest that OA might affect protein metabolism and nitrogen excretion in corals, however the mechanism remains equivocal.