A modified Bingham‐type rheometer, designed for operation at temperatures up to 500°F and at pressures up to 2000 p.s.i. is described. Interchangeable orifice plates permit wide variation of shear conditions. With this instrument flow properties may be studied under conditions approximating those encountered in the actual processing of thermoplastics, e.g., by molding or extrusion. Typical results are presented for cellulose acetate, polystyrene, and polyvinyl resin plastics, and correlation with practical experience is pointed out. The instrument is slow, and this limits its usefulness for other than research investigations.