Saplings of the tropical trees Tibouchina pulchra (Cham.) Cogn., Caesalpinia echinata Lam., and Psidium guajava L. cv. Paluma were exposed in open-top chambers with charcoal filtered air and measurements of gas exchange and chlorophyll fluorescence were made before (t1) and after exposure to non-filtered air plus O3 (t2), simulating 6-h peaks of O3 similar to those observed in São Paulo city (SE Brazil, reaching an AOT40 of 641 nmol mol-1). After the fumigation, the net photosynthetic rate, stomatal conductance, transpiration rate, and Fv/Fm were reduced (p<0.05) for the three species. C. echinata was the most sensitive species and P. guajava cv. Paluma the most resistant. and R. M. Moraes ... [et al.].