A 30-day ingestion of -y-hexachlorocyclohexane (lindane) by carp (Cyprinus carpio) induced hypoglycemia without activation oftwo hepatic gluconeogenesis enzymes (fructose diphosphatase, EC 4.1.2.13, and glucose-6-phosphatase, EC 3.1.3.9) and hyponatremia and variations in muscle plasmic membrane-bound enzymes (especially cholinesterases, EC 3.1.l.7). After 109 days carps exhibited a decrease in natremia but no significant hypoglycemia. There was an activation of gluconeogenesis enzymes. Important changes were observed in the activities of muscle plasmic membrane enzymes (especially 5'-nucleotidase, EC 3.1.3.5, and ATPases, EC 3.6.1.3). Lindane, a lipophilic substance, especially disturbed the activity of membrane-bound enzymes enclosed in a phospholipid matrix.