Estrous synchrony was tested using 10 pairs of sibling female rats (Rattus norvegicus). A Monte Carlo bootstrap simulation was used to construct random control groups to avoid previous statistical errors and to test for significance when there are irregular cycles. The 10 pairs of females did not exhibit estrous synchrony. The effect of cycle irregularity on the limits of synchrony was analyzed using an equation that the related degree of cycle regularity to the degree of synchrony. This equation significantly predicted the limits of synchrony: cycle irregularity limits both the maximum and minimum degree of synchrony that can occur between two females. Finally, simulations of the expected levels of synchrony in groups of five rats were compared to the original study on estrous synchrony. The simulations indicated that the results of the original study were consistent with chance levels of synchrony. It is concluded that there is no evidence that Norway rats synchronize their estrous cycles. Evolutionary implications are discussed.