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On the basis of previous research, we predict that Mediterranean grasslands should show larger-seeded annuals in: (1) more-arid grasslands; (2) more-fertile soils; (3) less-grazed grasslands; and (4) grasslands with lower intensities of seed predation by ants. To test these predictions, we set 29 sampling units of 50m x 50m in a 1000 km(2) grassland area in Central Spain, and characterized them according to the former factors. We then recorded annual vegetation using ten quadrats of 20 cm x 20 cm in each sampling unit. Seed size at the community level was described using six variables: (1) mean seed mass; (2) standard deviation of seed mass; (3) weighted mean seed mass (by species frequencies); (4) proportion of small-seeded annuals; (5) proportion of medium-seeded annuals; and (6) proportion of large-seeded annuals. Most climate variables (mean annual temperature, length of the summer drought, water balance and mean annual precipitation) correlated with seed-size descriptors, showing that large-seeded annuals increase in warmer and more-arid communities. Mean seed size was modelled as a function of mean annual temperature and grazing pressure. According to this model, warmer and less-grazed communities tend to show a smaller mean seed size. These results confirm the importance of seed-size descriptors at the community level in Mediterranean grasslands, and the role of climate and grazing as major drivers in these communities. Conversely, hypotheses about soil fertility and seed predation by ants were not supported by our results.