A rheometer device 31 for determining the rheological properties of blood is disclosed. It comprises: a channel 21, 22, 23 with a sub-section 21 of either constant or uniformly increasing or decreasing cross-section； apparatus for determining a pressure differential along a length of the channel sub-section； two reservoirs 11, 12 with variable internal volumes located at either end of the channel, the reservoirs thereby being in fluid communication with each other； a means for introducing blood into the device； an outlet allowing for the expulsion of gas； and means for varying the volume of one reservoir such that blood may flow along the channel in alternating directions. Each reservoir may comprise a plunger 11a, 12a movable along a hollow cylinder 11b, 12b, and may be either a syringe or an elastic body. There may be a plurality of channel sub-sections 21, 22, 23 of different cross-sections. The device may measure the rate of blood flow between the first and second reservoirs.
