European eels (Anguilla anguilla) were exposed to a sublethal thiobencarb concentration of 0.22 mg/L in a flow-through system for 96 h. Mg(2+) and Na(+)-K(+) adenosine triphosphatase (ATPase) activities were evaluated in gill and muscle tissues at 2, 12, 24, 48, 72, and 96 h of thiobencarb exposure. Gill ATPase activities were rapidly inhibited from 2h of contact onward. Highest inhibition was registered for Na(+), K(+)-ATPase (85%) from 2 to 12h. Both Mg(2+) and total ATPase were inhibited (>73%) during the first hours of toxicant exposure. At the end of the exposure period (96 h) ATPase activities were still different from those of the controls (>50%). Significant inhibition was detected in Na(+), K(+)-ATPase activity (80%) in muscle tissue after 2h and it was maintained over the entire exposure time. However, Mg(2+)-ATPase and total ATPase showed only perturbations after 2 h of exposure. Eels were exposed to 0.22 mg/L of thiobencarb for 96 h and then a recovery period in herbicide-free water was allowed for 192 h. Gill and muscle samples were removed at 8, 24, 72, 96, 144, and 192 h and ATPase activity was evaluated. Following 144 h of recovery, Mg(2+)- and Na(+), K(+)-ATPase activities, as well as total ATPase activity, in gills of those animals previously exposed to 0.22 mg/L of thiobencarb were still significantly different compared to controls. Thiobencarb seems to act to alter the ionic profiles. Since ion-dependent ATPases are known to regulate the influx and efflux of ions across the membrane to maintain the physiological requirements of the cells, the inhibition of Na(+), K(+)-ATPase probably induced osmoregulatory perturbations. On the other hand, thiobencarb exposure causes increases in the muscle water content of A. anguilla. The results indicated that water content increased significantly (>100% higher than the controls) during the first 24 h of exposure.