In this study we analyzed the effects of NaOCl solution on blood hematological and biochemical parameters of the rainbow trout Oncorhynchus mykiss (Walbaum, 1792). The treatment lasted for three days with prior dechlorination of the water. Significant differences in the concentration of K+, Ca2+, Cl-, proteins, glucose were detected and the largest variations were found for creatinine concentration (P<0.05). Significant differences were found for PCV, Hb concentration, RBC, WBC, MCH, MCHC, LYM (%), SEG (%) and NEU (%) values (P<0.05). Small doses of NaOCl alter the acid-base balance, suggesting a very low-level adaption in the rainbow trout.