A consequence of accumulating carbon dioxide (CO2) in the atmosphere is global climate change. CO2 also dissolves in the ocean and reduces the water pH, resulting in ocean acidification (OA). Fish were long thought to be relatively resistant to pH changes in the water, because they possess an efficient acid-base regulation. However, recent findings indicate altered behaviour and changes in neurological processes in fish. To investigate the acid-base regulation in the brain of the polar cod (Boreogadus saida), a combination of two methods, based on the nuclear magnetic resonance (NMR) phenomenon, was used in this study. Non- localized 31P-NMR spectroscopy was used to measure the concentration of phosphometabolites. The intracellular pH (pHi) was calculated from the chemical shift of the 31 NMR signal of inorganic phosphate in relation to the phosphocreatine signal. spectroscopy is used to detect short-term changes in the concentration of different phosphometabolites and hence short-term changes in the pH. However, the determination of the pH value in a specific region as small as the fish brain is not possible with non-localized 31P- NMR spectroscopy. To verify the results of the measurements the chemical exchange saturation transfer (CEST) between taurine and water (TauCEST) was determined in a specific region in the brain of B. saida. Because CEST is pH dependant, changes in the CEST effect can give evidence on any pH changes with a higher spatial resolution than non-localized 31P-NMR spectroscopy. Additionally to pH changes, energy metabolism can be analysed with 31P-NMR spectroscopy by measuring the concentration of phosphometabolites such as inorganic phosphate, phosphocreatine or the three ATP subunits α-, β- and γ-ATP. After 20 hours of acclimatisation under control conditions, the animals were exposed to a CO2 concentration of 3500 ppm and a water pH of 6.92 ± 0.2 for four hours (hypercapnia). Then, the animals were tested again in water without elevated CO2 concentrations (pH 7.96 ± 0.3). The pHi decreased rapidly after switching to hypercapnia by a mean of 0.05 ± 0.2 and started to reach control values again after two hours. The maximum decrease in pHi was 0.17 and occurred in fish 2 and 4. Throughout the whole time of the experiment, there were no significant changes in the energy values.