A quasi-static method is proposed for evaluating spatial effects on nuclear reactor kinetics. The neutron flux shape is calculated approximately as an asymptotic solution of the two-group space-time diffusion equations, where delayed neutron behavior is included. Two iterative procedures are alternatively used according to the amount of reactivity involved. The first one operates until prompt criticality is reached. The second procedure replaces the first one as soon as the reactor goes superpromptcritical. The main feature of the approach adopted is the possibility of selecting an initial guess such that convergence is reached at the first iteration. The matter is then reduced to solving two eigenvalue problems. Theoretical and numerical comparisons with Henry's adiabatic model outline the main role of perturbed adjoint fluxes and correct neutron-flux shape (the second agent only for superpromptcritical excursions) in defining the generation time and reactivity. When compared with the exact solution, results of sample problems show substantial accuracy in the flux shape and amplitude. In subpromptcritical excursions, only the synthesis method is as accurate as the metastatic one and yields errors of few percent at the flux peak. In the reactivity range above prompt critical, differences between the exact results and the metastatic ones are unessential.