The physiological responses of rainbow trout (Salmo gairdneri) to fenvalerate intoxication during aqueous exposure were examined to provide information about the pyrethroid mode of action in fish. Trout (n = 4) were exposed to 412 + or - 50 micro/liter fenvalerate and died in 10.9 + or - 1.5 hr. Brain, liver, and carcass fenvalerate concentrations associated with mortality were 0.16 + or - 0.05, 3.62 + or - 0.57, and 0.25 + or - 0.05 mg/kg, respectively. Visible signs of intoxication included elevated cough rate, tremors, and seizures. Histopathological examination of gill tissue showed damage consistent with irritation. An evaluation of respiratory-cardiovascular and blood chemistry responses indicated an elevated rate of metabolism associated with increasingly severe seizures. A cessation of ventilatory and cardiac activity, occurring with the seizures, was also observed. Finally, urine osmolality. Na- and K+ concentrations, and Na+ and K+ excretion rates were elevated with intoxicated trout. The physiological responses of rainbow trout to fenvalerate intoxication suggest that besides effects on the nervous system, effects on respiratory surfaces and renal ion regulation may be associated with the mechanism of pyrethroid action in fish. (Copyright (c) Pesticide Biochemistry and Physiology 27, 275-288, 1987.)