Anti-siphon devices (ASD) were initially bench tested at flow rates between 10 and 50 cm3/h and with the distal catheter height between 0 and -60 cm. There was a small increase in pressure with increased flow rate in the horizontal position (p less than 0.001). The inflow pressure initially dropped with the distal catheter height at -20 cm; it then rose progressively with distal catheter heights of -40 and -60 cm (p less than 0.001). To determine the effect of ambient pressure the devices were placed in a barometric chamber at pressures between -200 and +200 mm H2O. Positive pressures caused a linear increase in inflow pressure; negative chamber pressure reduced the anti-siphon effect. Eight ASDs were implanted subcutaneously in piglets and tested in situ weekly for 4 weeks. Implantation caused a mean increase in inflow pressure of 93.5 mm H2O 7 days after implantation (p less than 0.001) and which persisted for 4 weeks. Incision of the capsule surrounding the ASD at the end of 4 weeks caused a drop in pressure. The capsule consisted of an outer layer of collagen fibres with an inner layer of histiocytes. Subcutaneous implantation of ASDs causes an increase in the ambient pressure of the device which significantly increases their resistance to flow.