Dual lumen silicon rubber right atrial catheters were implanted into the jugular of 8 rabbits and tunneled subcutaneously to exit sites between the ears. Using a miniaturized tubing-pump system, blood flow rates of 25 ml/min could be achieved for up to 3 1/2 hours without sign of hemolysis in an extracorporeal blood circuit. Seven catheters functioned an average of 75 +/- SE 19 days (range 24-117). One catheter remains functional after 176 days. Infection and thrombosis were the main reasons for failure. 20 plasmaphoresis experiments were carried out in four heparinized rabbits (blood flow rates 15 ml/min, plasmaflux 1.5-2.0 ml/min) using polypropylene minifilters (average pore size, 0.55 micron) with the plasma recirculated back into the animal. No hemolysis was detectable throughout the 4 hr experiment. Plasma proteins with a MW of 69 X 10(3) to 3 X 10(6) (Albumin, LDH, SGOT, SGPT, CPK, fibrinogen, LDL) showed a sieving coefficient close to 1.0. The good filtration performance and the absence of side effects make this system a possible use for plasmaphoresis in neonates.