Abstract: The paper describes some of the early changes in blood of the grass carp following artificial infection of Reovirus. The activities of serum lactic dehydrogenase (SLDH) isozyme of Reovirusinfected fish were compared with normal individuals. Hematological changes of fish naturally suffering from this disease were studied at the epizootic area. All data obtained were treated statistically.Hematological and blood chemical changes were examined after fish were infected artificially by Reovirus. Significant reduction of red blood cell(RBC), total plasma protein and urea nitrogen occurred in the latent period by 4—5 days, although there were no obvious symptoms. When entering the developing period by 8 days, fish were moribund with typical hemorrhagic symptom, with RBC, hemoglobin (Hb) and total protein reduced even more obviously. However the urea nitrogen of the diseased fish was still maintained at the level of latent period. The plasma glucose concentration also changed insignificantly. The relative activity of SLDH isozyme seemed to be deranged during the latent and developing periods of the disease. In the control fish, its order was SLDH1SLDH2SLDH3SLDH4SLDH5 but in the infected fish, the order was reversed into SLDH1SLDH2SLDH3SLDH5SLDH4SLDH6, in which the last band (SLDH6) was detected near negative pole on the cellulose acetate membrane electropherogram of some infected fish.The early hemopathologic changes in fish occurred during epizootic period. Two groups of fish were examined. One group consisted of fish with pronounced symptom, and the other group was taken randomly from the same fish pond. Results showed that changes in RBC, Hb, total protein, glucose concentration, urea nitrogen and SLDH isozyme occurred in both groups and were similar to those found in artificially infected fish. Moreover, there was a marked increase in the level of serum potassium and an obvious reduction of serum calcium. Correlation between body weight of infected fish in the latent period and various blood parameters was analyed. It showed that the contents of RBC, Hb, total protein, glucose concentration, urea nitrogen, albumen were negatively correlated to body weight, But the relations of body weight to the levels of serum potassium and calcium were inconspicuous.