The time dependence of stress in simple extension of polyisobutylene (viscosity‐average molecular weight2.3×106) was measured for extension at various rates to a stretch ratioλ=1.5followed by relaxation at constant λ. The course of relaxation was independent of initial stretching rate after a time interval of approximately three times the time required to reachλ=1.5.In a series of partial relaxation experiments at stretch ratioλ0=2.0,the stress was allowed to relax for periods from 100 to1.3×105 secand the sample was then released by cutting; the recovery after a time interval δ of either 5 or 15 sec was measured. The stretch ratioλscorresponding to this point of initial recovery agreed closely with that calculated on the basis of a two‐network model in which an entanglement network with reference stateλ=1is gradually replaced by another with reference stateλ=λ0during relaxation, while the total concentration of entanglement strands remains constant. The old network (in extension atλs) is described by a Mooney‐Rivlin formulation and the new network (in compression atλs) is neo‐Hookean. The Mooney‐Rivlin CoefficientsC1(t)andC2(t)required for the calculation are obtained from parallel experiments of stress relaxation at various stretch ratios.