The application of generalized perturbation theory, both time dependent and static, to burnup problems is discussed. Simple analytical forms, applicable for many cases of interest, are derived for the time-dependent expressions. The influence of changes in Φ(E,r) during the cycle on the production of the plutonium isotopes is calculated for a typical liquid-metal fast breeder reactor case, using perturbation theory. Comparison of the results with direct calculations indicates that perturbation theory gives satisfactory accuracy for this problem.