1. Carp were exposed to 280 ppm formalin. Haematocrit and plasma glucose and lactic acid increased. In moribund fish, blood pH was remarkably lower and plasma protein increased. 2. When exposed to formalin, heart rate (HR) and respiration increased briefly, and then decreased. 3. Some fish increased urine flow (UF) immediately and maintained higher values for a while, followed by gradual decrease, and others decreased UF consistently. UF stopped at 30 min or longer prior to fish death. Urine osmotic pressure was higher at the 1st to 2nd hour and immediately before UF stopped. 4. The relationship between UF, HR and respiration was also discussed.