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The long time relaxation of stress and diffusion in polymer melts and semidilute solutions are well described by de Gennes’ s1 and Doi-Edwards’2 reptation model. However, at times shorter than the chain’s Rouse time tr, the model predicts an intermediate “breathing” regime, 2-3 where a chain segment (n) follows anomalous Rouse diffusion along the tube,((/„) 2)« 1/2 (with ln denoting the segment position along the tube). Since the chain is Gaussianly distributed in space ((r2)«= 0, the real-space displacement of a chain segment, in particular that of the chain ends, follows2-3