A method and device for reducing blood coagulation in the circuit of a device for substitution of the kidney function wherein, prior to the input in the device for substituting the kidney function, the blood which left the patient's body is cooled down to a temperature in the range 10.degree. C. to 30.degree.
C. and, as the blood passes the device for substitution of the kidney function, that blood is warmed up to a temperature at least near to the body temperature and then it is returned in the patient's body.