An ambulatory ultrafiltration device includes a blood filter that has a blood side or fluid communication with the vascular system of the subject, an ultrafiltrate side, and a semipermeable membrane disposed between the blood side and the ultrafiltrate side. The ambulatory ultrafiltration also includes a buffer vessel in fluid communication with the blood side of the blood filter, and a blood pump. The blood pump is controlled to alternate between a withdrawal phase and a return phase. In the withdrawal phase, blood is withdrawn on a blood path from the subject via the blood filter to the buffer vessel. In the return phase, blood is returned from the buffer vessel to the subject on the blood path. The blood filter is arranged to remove ultrafiltrate from the blood during at least one of the withdrawal and return phases.