A common assumption is that increasing transmembrane pressure by lowering the ultrafiltrate receptacle and the accompanying fluid column should always result in increasing ultrafiltration in continuous arteriovenous hemofiltration (CAVH) systems. To test this assumption, CAVH circuits were operated in vitro with use of a recirculating apparatus with an adjustable elevated reservoir. Hydraulic operational characteristics were studied in a variety of CAVH circuits, lowering the height of the ultrafiltrate column stepwise until it was at the lowest height possible. The results for most experiments performed reveal that ultrafiltration rate (UFR) reaches a peak and then declines as the collection receptacle is lowered further. There is also a decline in pre-filter blood flow preceding the peak in UFR. At lower blood flow, UFR decreases for the same transmembrane pressure (TMP). Therefore, as ultrafiltrate pressure is decreased, the effect of increased TMP on UFR is opposed by the effect of decreased blood flow, which decreases UFR. The implication of this in clinical medicine is that one may need to empirically test UFR in a CAVH system in positions other than the very lowest position.