Stress relaxation measurements were made as a function of temperature and hydrostatic pressure on two lightly filled elastomers (Hypalon 40 and Viton B), one highly filled elastomer (Neoprene WB), and on an EPDM rubber. The latter was not piezorheologically simple. The lightly filled elastomers showed piezorheologically simple behavior, i.e., their response curves under different hydrostatic pressures could be superposed empirically by a simple horizontal shift along the logarithmic axis. The filled elastomer was piezorheologically simple only in the rubbery region and in the beginning of the transition region. The dependence of the empirical shift distances,log ap,onPcould not be described by either the Ferry‐Stratton or the Bueche‐O'Reillyequation. By considering the bulk modulus to be linearly related to pressure, a new equation has been developed forlog aT,Pwhich describes the pressure dependence well and contains the WLF equation as a limiting case. Published data on the response of poly(vinyl chloride) under superposed hydrostatic pressure are shown to obey the new equation also. The theoretical importance of the new equation lies in the fact that combination of the usual isobaric measurements at atmospheric pressure as function of temperature with isothermal measurements as function of pressure allows, in principle, all the molecular parameters required by the free volume theory to be determined unambiguously.