The paper discusses the modifications produced in some biochemical indices as well as in the concentration of serum ions, in fry carp, during the period of active feeding, in two experimental ponds of the Iasi district, providing different conditions of ichtyo-pathological prevention. The average values of glucose and alkaline phosphatase from the blood, as well as those of cholesterol, ureic nitrogen, Na+, Ca2+ and K+ from the serum, have been determined after 120 days of experiment (June - September), for 11 representatives of each basin. For the two experimental variants, different preventive treatments - based on calcium chloride administered in three different moments of the growing cycle and on concentrated, drug-containing food for assuring antimicrobial prevention - have been applied. The values recorded in the variant under analysis are lower than those of the reference, with 12.2% for glycemy, 9.7% for total cholesterol, 17.5% for the total ureic nitrogen and 10.98%, respectively, for alkaline phosphatase. The levels of the determined ions show higher values in the experimental variant, with 23.3% for K+ and 4.1%, respectively, for Ca2+, while the average values for Na+ are lower in the fish from variant B, the Na+/K+, Na+/Ca2+ and (Na++K+)/Ca2+ ratios showing inferior values - with 4 - 21% - than the reference ones.