Abstract: The paper is the second part of haematolocal studies on the grass carp (Ctenopharyngodon idellus) fingerlings. One year old fishes were obtained from the Fish Farm of Institute of Hydrobiology, in March, 1980, and February, 1981, Altogether 454 healthy fishes were used for clinical tests. At the time of sampling, the fish on average measured 17.15cm (10.1-26 cm) and weighed 56.22g(10.3-200g). Concentrations of sodium, potassium, calcium, chloride, inorganic phosphorus and urea nitrogen in serum, their monthly variations and factors affecting these parameters were described in the present paper. The annual mean concentrations of serum electrolytes and urea nitrogen of the fingerlings were: sodium, 122.31 mmol/L (S.D.+14.44); potassium, 4.17 mmol/L(S.D.?.36); calcium, 2.07 mmol/L(S.D.?.42); chloride, 89.3 mmol/L (S.D.?2.97); sodium chloride, 518.66mg/l (S.D.?1.90); inorganic phosphorus, 7.84mg/l(S.D.?.22). Though contents of serum electrolytes and urea nitrogen fluctuated within a year, such fluctuations did not show any clear patterns. At high water temperatures, concentration of potassium increased and those of sodium and calcium decreased. But levels of chloride, inorganic phosphorus and urea nitrogen remained constant. In addition, contents of sodium and urea nitrogen increased, and potassium content decreased with increased body weight. Contents of calcium, chloride and inorganic phosphorus did not vary with both weight.