The effects of drought and heat stress on physiology of two common cotton species, Gossypium hirsutum L. (Upland) and G. barbadense L. (Pima) were investigated in this study. Four consecutive 28-day pot experiments were carried out in a fully controlled growth chamber under four temperatures (25, 30, 35 and 40 degrees C), while two water treatments (well-watered and drought) were imposed for 10 days to both cotton species. Growth according to dry matter accumulation of G. barbadense was more limited by sole drought conditions (50%), whereas heat stress was more remarkable on growth of G. hirsutum (64%). Chlorophyll (Chl) a and b content were more temperature-sensitive in G. hirsutum than G. barbadense under well-watered conditions. The most noticeable decrease in drought-induced rates of photosynthesis rate (P-n), transpiration rate (E) and stomatal conductance (g(s)) were recorded under 35 degrees C, although genotypic variation was found under 25 degrees C. Higher water use efficiency was associated with higher temperature. Relative decrease in Chl a content, g(s) and maximal quantum efficiency of PSII exhibited by both species were distinctive physiological traits for heat and drought tolerance.