Abstract: A sampling apparatus is provided comprising a pump having duplex independently valved first and second inlets and a common discharge outlet, the first inlet being adapted to be connected to the vascular system of a patient via an indwelling catheter or like semi-permanent connection while the second inlet is adapted for connection to a source of normal saline. A series of collector vessels is provided for receiving fluid discharging from the pump discharge outlet. The apparatus is operated in accordance with a preselected program whereby at prescribed intervals the pump is caused to withdraw a blood sample of chosen volume through the first inlet and to discharge it to a collector vessel and thereafter to draw saline from the second inlet to flush the pump while discharging the flushings to a separate collector vessel. Preferably, the collector vessels are disposed in a rotary indexing carrier adapted to bring each collector vessel in turn into position under the pump discharge outlet.