Questions: Does seed mass influence the tolerance of seeds to the effects of heating in fires? Is the tolerance to heat shocks during fire events dependent mostly on seed mass itself or to other traits linked to the species ecological origin, e.g. non-fire-prone (forest) and fire-prone (savanna) environments? Location: Savanna and forest ecosystems of the Cerrado biome, Brazil. Methods: Heat shocks were applied to seeds of 17 selected common tree species in savanna (ten species) or forest ecosystems (seven species) of the Cerrado biome. After being submitted to a heat shock, seeds were allowed to germinate in chambers set at 30 °C. Germination percentage was calculated for each treatment and species. For comparison purposes, species were grouped into two functional groups (forest and savanna) and also into seed size classes (small, medium and large). We used generalized linear model to analyse heat shock effects on seed germination of savanna and forest tree species, and how seed size affected the germination response. Results: Seed mass influenced seed tolerance to heat shock. Larger seeds had higher survival when subjected to heat shock, independent of species' ecological origin. In addition, seeds from species occurring in fire-prone environments (savanna) were more tolerant to high levels of heat shock than seeds from species occurring in non-fire-prone environments (forest). Conclusion: In response to global climatic change, fire regimes in savannas of South America are expected to become more frequent and intense, even expanding into currently non-fire-prone environments. In this sense, our findings indicate that species from savannas (fire-prone environments) might have an adaptive advantage to these consequences of climatic changes, potentially leading to expansion of the savanna ecosystems.