A simple system for the continuos withdrawal of blood and the serial collection of blood samples in human subjects and experimental animals is described. It consists of heparin-coated tubing, a connector attached to an indwelling cannula that allows flushing of the i.v. catheter, a peristaltic pump and a fraction collector and an alarm system which is activated when blood flow through the system is interrupted. It can be assembled from parts available in every laboratory or hospital.