Abstract: Apparatus for carrying out peritoneal dialysis. The apparatus comprises a weighing device (37) provided with a case (50) for forming an airtight space. A heat bag (38) and, optionally, a discharge bag (42) are positioned in the space, said bags being connected with a catheter for the patient, who is to perform peritoneal dialysis. The apparatus comprises a pump arrangement (57) connected with the case (50) for producing overpressure and underpressure, respectively, in the space inside the case. Valve arrangements (39, 44) are adapted to connect either the discharge bag (42) or the heat bag (38) with the patient's catheter. By providing an underpressure in the case (50). fresh dialysis solution is provided to the heat bag (38) from supply bags, whereupon used dialysis fluid is extracted from a patient to the discharge bag (42).