A heterogeneous method of calculating time-dependent reactor core characteristics, such as the time variation in thermal-neutron flux distribution and the reactivity change during fuel and poison burnup, is derived. The lattice consists of an infinite number of similar square zones closely connected to one another. In each zone, identical fuel rods are arranged in a regular lattice with a burnable poison rod of the same geometric dimensions as a fuel rod at the center. Some numerical examples, utilizing the equations derived finally, give the time variation in poison concentration and k∞(t) for a zone showing the heterogeneity effect associated with a burnable poison rod. Since the machine time required to compute the time variation of such core characteristics through fuel life of 11,000 EFPH as shown in the examples is <25 sec on the IBM 360/75 per case, the method may be applied to the preliminary survey calculation for the time-dependent heterogeneous core characteristics of a square lattice including burnable poison rods as well as to more general time-dependent problems related to such lattices.