The following theoretical equation has been obtained for measuring the rate of slow crack growth in polyethylene in terms of the crack opening displacement rate delta : delta=sigma(y)(1-gamma(2))(2)/eta d(0)E(2) sigma(c)(2) K-4 Here sigma(y) is the yield point, K is the stress intensity, eta is the intrinsic viscosity of the fibrils in the craze, E is Young’s modulus, sigma(c) is the stress to produce a craze, d(0) is the primordial thickness from which the craze originates and gamma is Poisson’s ratio. The theoretical equation agrees with the experimental observation : delta-CK(4)e(-Q/RT) Thus, for the first time, the dependence of delta on stress and notch depth have been derived in fundamental terms and the physical parameters that constitute the factor C have been identified. The intrinsic viscosity eta can be calculated from the theory using specific experimental data. For example at 42 degrees C, the fibrils in a craze in a homopolymer have an intrinsic viscosity of 3 x 10(11) Pas. This is much larger than the melt viscosity of the amorphous region, which is about 10(5)-10(6) Pas. Thus, the resistance of polyethylene to slow crack growth is governed by the crystals and not by the amorphous region.