A sealed compartment recycling system has been created for use with Extracorporeal commercial filters in order to exploit the negative pressure of the dialysate for purposes of ultrafiltration. The results of in vivo and in vitro tests regarding ultrafiltration and dialysates of urea, creatinine, Hipaque I125 and vit. B12 Co57 are reported. The tests highlighted improved dehydration characteristics in the system compared with the traditional coil technique, while the dialysates of small and medium molecules showed no decrease. The system can profitably be used as an alternative to the open compartment coil system and is particularly interesting because it can be combined with monitors which provide exclusively for the use of closed circuit dialysate instruments.