Abstract: A sampling system for a blood treatment circuit is disclosed, and is designed to collect a sample representative of all waste liquid discharged from a blood treatment apparatus during a treatment session. The circuit includes a feed line for supplying fresh treatment liquid to the blood treatment apparatus and a discharge line for evacuating waste liquid. A withdrawal line connects the discharge line to a collecting container, and a flow meter in each of the discharge line and the withdrawal line measures liquid flow. A controllable flow restrictor such as a valve is provided in the withdrawal line. A controller selectively opens the flow restrictor each time a predetermine volume is measured in the discharge line and closes the flow restrictor each time another predetermined volume is measured in the withdrawal line.