A novel type of constant speed reciprocating pump with a positive displacement action is described. The pump consists of a pumping chamber with non-return valves to prevent back-flow, and a smoothing chamber where the pressure and flow pulsations are reduced in amplitude. The pump runs at 95 strokes/min and 190 strokes/min. The rates of flow, continuously variable by altering the stroke volume, cover the ranges 0–2500 ml/min and 0–5000 ml/min, respectively. The complete unit includes a reservoir, line filter and interconnecting tubing. The rate of damage to blood elements is small; consequently the apparatus can be used for perfusions of long duration.