The effect of heat treatment on domoic acid (DA) content in soft tissues of mussels Mytilus edulis was investigated using high performance liquid chromatography. DA concentrations in whole flesh, hepatopancreas and tissue remainder were measured in fresh, steamed and autoclaved mussel flesh. Relative decreases in DA and tissue fluid following heat treatments of whole flesh were similar resulting in approximately equal concentrations of DA pre- and post-treatment. DA concentration decreased in the hepatopancreas and increased in tissue remainder suggesting some organ disruption of mussels during heat treatment. These findings suggest that heat treatments using either conventional steaming or autoclaving at 121 degrees C are not appropriate techniques to reduce DA in mussels during commercial processing. We also conclude that sample pre-treatment has a minimal effect on the result of a DA analysis on whole mussel tissues. The stability of DA at different temperatures within a shellfish matrix was separately tested. Reductions in DA concentration (ca. 3-7%) compensate for some of the discrepancies between what was found in the cooking fluids in the initial study and what was expected based on the whole flesh concentration of the uncooked material.