The relaxation time (T-e) associated with the transition from liquid-like behaviour to pseudo solid-like behaviour for polybutadiene and polyisoprene, as observed in simple Bloch decay n.m.r. experiments, is shown to be linearly dependent upon the polymer molecular weight. This transition begins at the critical molecular weight, M-c, as classically defined by a variety of rheological experiments. The linear dependence of T-e upon molecular weight, using a simple diffusion model, requires the length scale over which the relaxation is operative to be independent of total polymer molecular weight. The experimental results identify the length scale associated with this relaxation time to be the length scale represented by the critical molecular weight (M-c); this process is observed to utilized are found to be sensitive to chain dynamics on the length scale corresponding to the critical molecular weight, not the total polymer molecular weight. (C) 1997 Elsevier Science Ltd.