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In view of anthropogenic global warming, heat tolerance of a neotropical pioneer tree, Ficus insipida Willd., was determined. Sections of sun leaves from a mature tree and from seedlings cultivated at ambient and elevated temperatures were heated to 42-53 degrees C. Leaves from a late-successional tree species, Virola sebifera Aubl., were also studied. Widely used chlorophyll a fluorescence methods based on heat-induced rise of initial fluorescence emission, F-o, and decrease in the ratio of variable to maximum fluorescence, F-v/F-m, were reassessed. F-v/F-m determined 24 h after heat treatment was the fluorescence parameter most suitable to assess the lethal temperature causing permanent tissue damage. Thermo-tolerance was underestimated when F-o and F-v/F-m were recorded immediately after the heat treatment. The limit of thermo-tolerance was between 50 and 53 degrees C, only a few degrees C above peak leaf temperatures measured in situ. The absence of seasonal changes in thermo-tolerance and only marginal increases in thermo-tolerance of plants grown under elevated temperatures suggest little capacity for further heat acclimation. Heat-stress experiments with intact potted seedlings also revealed irreversible leaf damage at 51-53 degrees C, but plants survived and developed new leaves during post-culture.