We studied thermal denaturation of myofibrillar proteins from pre-and post-spawning hake by differential scanning calorimetry (DSC), and evaluated denaturation kinetics under both conditions. The denaturation enthalpies of all pre-spawning fish muscle extracts were less than those from post-spawning. The area under the DSC thermogram corresponding to myosin denaturation was smaller in myofibrillar extracts from pre-spawning than from post-spawning hake, while the areas corresponding to denaturation of actin were similar. Between 40 and 55°C the myosin denaturation rates were greater for post-spawning than for pre-spawning hake. Both entalphies and kinetic data indicated proteins of fish in a better biological condition (post-spawned) denature more rapidly and completely.