A model in which an extended dislocation jumps about a pinning agent (discussed previously by Blair et al.) gives rise to damping and modulus defect terms. These are calculated for dislocation loops of constant length and for an exponential distribution of loop lengths. The variation of damping and modulus defect with applied cyclic stress and with temperature are deduced and compared with the behavior of a Granato–Lücke breakaway model. The above terms are also calculated for the general problem in which a system "jumps" between two potential wells.