The metabolism of algal toxins by marine organisms is a field of emerging interest. In spite of numerous studies on the depuration of paralytic shellfish toxins (PSTs) from marine bivalves, little work has been conducted on the mechanisms of PST detoxification. Previous research indicates that exposure of salmon to saxitoxin by intra-peritoneal (ip) injection increased cytochrome P-4501A activity in the liver. Similar exposure experiments show PSTs to cause induction in the activity of the phase II detoxification enzyme, glutathione S-transferase (GST) in salmon (Salmo salar) livers and mussel (Mytilus edulis) digestive glands. Liver samples from salmon periodically injected ip over 21 days with saxitoxin and a toxic dinoflagellate extract showed elevated GST activity relative to controls. Immunodetection of GST isoenzymes by western and dot blotting with mammalian antibodies suggests that increased expression of GST protein may be the primary cause of the activity induction observed. Mussels were also exposed to doses of saxitoxin by intra-muscular injection. A small but significant elevation of GST activity was noted in the digestive glands of exposed groups relative to controls. This activity induction may be due to increased GST expression or enzyme activation. This work suggests that there may be a role for detoxification enzymes such as glutathione S-transferase in the detoxification and elimination of PSTs from these fish and shellfish models.