The energy relaxation time &tgr;einn‐InSb for the temperature range 1.5 to 20°K has been measured directly in a heterodyne experiment at 69 GHz. A comparison of experimental values for &tgr;ewith theoretical values calculated for a degenerate electron gas yields a deformation potential constant and a piezoelectric potential constant. Values of 16.2 eV for the deformation potential constant, 1.7 × 104d1/2/cm for the piezoelectric constant, and 280°K for the optical phonon temperature give good agreement between experiment and theory when screening is neglected. The effects of electron degeneracy and screening upon energy relaxation are discussed.