Fish freshness was assessed with an enzymatic method using a polarographic probe at 0.7 volt (platinum vs silver) to monitor degradation of inosine monophosphate (IMP), inosine (H × R), and hypoxanthine (Hx). The K-value, a ratio of H × R + Hx/IMP + H × R + Hx, was determined in Sockeye salmon, Pacific cod and Pacific herring in ice and at 12°C. Results were compared with those from HPLC and sensory scores. K-values by polarography correlated well with HPLC. Loss of freshness accompanied rise in K-value in all studies. Sensory scores correlated relatively well with K-values in Sockeye salmon and Pacific herring but less with Pacific cod. K-value did not reflect eating quality of Pacific cod.