Abstract: A device for heating blood for transfusion is disclosed which has two vertically disposed opposed electrical heating plates which engage the opposite faces of a flat bag of flexible plastic material delimitating a fluid path therethrough which has a width progressively increasing from a lower inlet pipe for connection to a blood supply to an elongated single rectilinear main portion of uniform width and progressively decreasing from the elongated main portion toward an upper outlet pipe for connection to injection means. A flow restricting means is disposed therein on an intermediate level between the inlet pipe and the main portion of the fluid path to restrict the travel of fluid introduced by the inlet pipe and produce a substantially homogeneous flow all across the main portion of the fluid path. The heating plates include heat radiating fins or corrugations to reduce thermal inertia as the rate of blood flow changes.