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Biological and methodological factors influence the upper thermal limits (UTL) of ectothermic animals, but most factors have been studied independently. Few studies have integrated variables, so our understanding about sources of UTL variation remains fragmentary. Thereby, we investigated synergic effects of experimental protocols (heating rates, Delta Ts) and biological factors (ontogeny and body mass) on the UTL on the larvae of two anuran species (Physalaemus nattereri and Boana pardalis), specifically their Critical Thermal Maximum (CTmax). The species displayed slightly different responses to Delta Ts: In B. pardalis tadpoles both average and variance of CTmax increased at a fastest Delta T, the same response happened in P. nattereri tadpoles at slow and moderate Delta Ts. Also, the CTmax of P. nattereri declined at the end of metamorphosis independently of Delta T, but tadpoles at all developmental stages still displayed higher heat tolerance at the slow Delta T. Finally, we detected small, synergic effects of body mass and Delta Ts on the CTmax of both species. In small B. pardalis tadpoles and premetamorphic P. nattereri tadpoles, body mass had a positive effect on CTmax but only at slow and moderate Delta Ts, probably indicating physiological responses. A similar trend was observed in large B. pardalis tadpoles at the fast Delta T, but this result is likely to be influenced by thermal inertia. Our findings contribute to integrate the understanding of factors influencing UTL in small ectothermic animals. This understanding is critical to discuss the physiological component of vulnerability to climate change that is related to acute temperatures.