In this study the toxic effects of Nodularia, a cyclic pentapeptide hepathotoxin, on plasma enzymes including aspartate aminotransferase (AST) and lactate dehydrogenase (LDH) were studied. Nodularia is produced by Cyanobacterium Nodularia spumigena which is mainly found in salt and brackish waters. Salmo trutaa caspius were exposed to Nodularia in two different doses (1 × 10⁴ and 1 × 10⁸ cells.ml^-1) for 12, 24, 48 and 96 h and then fishes transferred to clean water for 8 days. LDH enzyme activity indicated significant differences at different times in high concentration and showed significant differences than control group in 12, 24 and 96 h after exposure to toxin in the high-dose group and in 12 h post exposure in the low-dose group. LDH enzyme activity in the high-dose group significantly higher than low-dose group in 12 h post exposure. AST activity indicated significant differences in the high-dose group at the different times, but this changes were not significant than control group. No significant differences were observed in recovery phase. This study indicated that LDH activity is a more appropriate indicator than AST for effects of exposure to Cyanobacterium N. spumigena in this fish.