In the present work we reports on investigations of affecting of guppy fishes (Poecilia reticulate) by silver ions (0, 0.3, 0.6, 1.2, 2.5 and 5 uM) under well controlled experimental conditions for seven days. To follow the physiological changes we attempted to determine metallothionein (MT) as a biomarker of heavy metal stress. For these purposes we suggested a new sensor utilizing carbon electrode with flow injection analysis. Primarily, we optimized the suggested sensor. The experimental conditions 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 uA. Under these conditions the detection limit was estimated as 100 nM of MT. After the optimizing step, the analysis of fish tissues followed. Based on the results obtained, MT concentration 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 reference technique.