Urine properties of carp in summer season were examined at regular intervals for 3 days. As listed below, each property changed incessantly but within a constant range, even under the near-natural conditions for fish (no restriction in water) used in this study. Urine flow 8.37±1.57ml/kg body Weight/h; osmolarity 29.4±1.2 mOsm/l; pH 7.23±0.06; inorganic ion con-centrations (mEq/l), Na⁺ 12.05±0.68, K⁺ 1.40±0.28, Ca²⁺ 0.54±0.22, Mg²⁺0.23±0.06 and; nitrogen compound concentrations (mg-N/dl), ammonium 10.71±1.77, urea 0.72±0.11, uric acid 0.57±0.01, creative 9.0±0.4, creatinine 1.0±0.3 and protein 1.36±0.36 (each of these data was expressed as the average value±standard deviation, using 5 test fishes).