In an effort to better define the timing of metabolic processes that underpin the critical swimming speed in fish, we developed a method for monitoring high-energy phosphates using 31P-NMR spectroscopy in Atlantic cod (Gadus morhua) during a Brett-style critical swimming speed (Ucrit) test. We were able to show that as Atlantic cod approached Ucrit, there was a decrease in phosphocreatine (PCr), concomitant with a significant increase in inorganic phosphate (Pi). At the same time, intracellular pH (pHi) dropped and the respiration rate (MO2) increased exponentially. These effects were graded and typically started between 30 and 60 minutes before Ucrit was reached, just as a kick and glide style of swimming was initiated. At Ucrit the fish were kicking exclusively. The pHi and the PCr decreases, as well as the Pi increase were all maximal at Ucrit. Recovery after Ucrit was characterised by an initially high MO2 that decreased exponentially, coupled with a low pHi that also increased exponentially to basal values and a decrease in the Pi signal. All had returned to basal levels within 3hr. Our experiments show that gait transition to kick and glide swimming is concurrent with the onset of anaerobiosis. The excess post-exercise oxygen consumption is similarly concurrent with the removal of anaerobic end-products and the formation of PCr.