Genotype × environment interaction plays an important role in the identification of genotypes with high and stable yields. The environmental components of such interaction can be attributed to location (spatial) and year (temporal) factors. This study considers a specific aspect of the location factor, i.e. that locations not geographically close to each other are likely to vary in terms of the conditions that determine the effect of the year factor, even during the same calendar year. In these situations, the year effects are considered to be random variables within each location. This study addresses the question of whether genotype × location interactions are consistent (or repeatable) over time: a question which is crucial when defining the target environments in a plant breeding programme and when deciding the optimum environment for selection. Using three sets of data from field experiments, various computation methods were developed to estimate variance components; the methods and estimates are presented here. Using the bootstrapping procedure to sample one year from each location within a given calendar year, we obtained bootstrap replicated values of genotype × location interaction and their summary statistics, including the coefficient of variation (CV). We suggest that the CV can be used to measure the repeatability of genotype × location interactions. A Genstat program that can be used to perform the required computations is available from the authors upon request.