The effects on kernel weight of removing all spikelets except the central 4 and of removing flag leaves were measured in two years in the field on 33 modern cultivars in Belgium and on a collection of 25 genotypes including old and modern genotypes in Sweden. Removing spikelets increased the weight of the remaining kernels in most varieties in all years. The responses differed among varieties and were not consistent among years. Kernel weight responded more to defoliation or spikelet removal in modern than in old varieties. Although potential kernel weight (defined as kernel weight in ears reduced to their 4 central spikelets) was strongly affected by seasonal effects, it was less subject to environmental variations than was kernel weight. Kernel weight response to spikelet trimming or defoliation was not related to source characters or to yield/ha. Low but significant correlations of potential kernel weight with ovary size at anthesis suggest kernel weight to be at least partly sink-controlled.