The pressure coefficient of viscosity of poly(alpha-methylstyrene-co-acrylonitrile) was measured using a high-pressure sliding plate rheometer (HPSPR) and two types of capillary rheometer: a piston-driven device with a throttle at the exit [ piston capillary rheometer with throttle (PCRWT)] operated at a fixed flow rate, and a counter-pressure nitrogen capillary rheometer (CPNCR) operated at a fixed pressure drop. In the HPSPR, the pressure, shear rate, density, and viscosity are all uniform throughout the sample, while the analysis of capillary data is complicated by the axial pressure gradient and the radial shear rate gradient. The polymer was found to be piezorheologically simple, and the HPSPR data indicated that the pressure coefficient of viscosity beta = dln(a(P))/dP decreased slightly with increasing pressure at high pressure. While beta from PCRWT data from different laboratories and instruments agreed fairly well, the beta values were on average about 2/3 of that from the HPSPR. The CPNCR yields beta about 18% lower than that of the HPSPR.