In this paper, we report on an investigation of affecting guppy fishes (Poecilia reticulata) by silver ions (0, 0.3, 0.6, 1.2, 2.5, and 5 muM) for seven days under well-controlled experimental conditions. To observe the physiological changes, we attempted to determine metallothionein (MT) as a biomarker of heavy metal stress. For this purpose, we proposed a sensor utilizing a carbon electrode coupled with flow injection analysis. The experimental conditions, which have been optimized, were as follows-applied potential: 750 mV, mobile phase: Britton-Robinson buffer (pH 1.9) with flow rate of 0.6 ml/min, time filter: 2.5 s, "current R": 1 muA. Under these conditions, the detection limit of MT was estimated as 100 pM. After the optimizing step, the fish tissues were measured. Based on the results obtained, MT content increased with increasing dose of silver ions and time of the treatment. The results obtained were in good correlation with those obtained by adsorptive transfer stripping technique coupled with differential pulse voltammetry Brdicka reaction, which was used as the reference technique.