Complete velocity-time (v-t) relationships for a pendulum striker during impact testing of sharp-notched high-density polyethylene have been recorded by means of laser-Doppler velocimetry. The velocity data have been processed using a microcomputer to give force-deflection curves and hence absorbed energy. Toughness is expressed in terms of the critical strain energy release rate, Gc, and the critical stress intensity, Kc. The plane stress Gc from pendulum energy was 11.3 kJ/m², and that from areas under force-deflection curves was 9.9 kJ/m². The authors observed that an initiation Gc of the order 2.3 kJ/m² might be expected for this material below its glass transition. By defining crack initiation in these room temperature tests as the point of maximum recorded force, an initiation Gc of 2.0 kJ/m² is obtained. The critical stress intensity factor, Kc, is also defined from the maximum force, and a value of 2.1 MN/m^3/2 is calculated. The elastic modulus is computed from compliance measurements to be 1.84 GN/m², whereas from E=Kc2∕G the plane-stress modulus is 1.93 GN/m², and the introduction of Poisson's ratio gives a plane-strain modulus of 1.69 GN/m².