Diumal changes in chlorophyll (Chl) a fluorescence characteristics of sun-exposed and shade leaves of some tropical and temperate tree species growing under fíeld conditions on sunny and cloudy days during summer were recorded. Significant reduction in photochemical efficiency of photosystem (PS) 2 measured as ratio during periods of maximum irradiance was observed in sun-exposed leaves of some plants. The extent of this reduction was probably a function of the plant species. Among the tropical species tested, Boehmeria rugulosa, Celíis ausíralis, Dalbergia sissoo, Eugenia jambolana and Grewia optiva showed maximum reduction (15-22 %), whereas species like Ficus racemosa and Sapium sebiferum showed least dechne (< 7 %) in the F^/F^ ratio. On the other hand, no significant reduction in this ratio was observed in Bauhinia purpurea. Temperate species, v/z. Acer oblongum and Aesculus indica, were most susceptible to high irradiances. Prolonged exposure of these plants to high irradiances resulted in lesion development in their leaves characterized by reduction in pigment contents and Fy/F^, ratio. Plant species with considerable midday dechne in Fy/Fn, ratio showed (7) slight but significant rise in initial fluorescence (Fq), (2) pronounced decrease in F^ value, and (3) significant reduction in area over the curve between Fq and Fj„ indicating the pool size of electron acceptors on the reducing side of PS2. The depression in Fy/F^ ratio though reversible could not be alleviated even after watering the plants at hourly intervals. No reduction in Fy/F^, was observed in shade leaves of the same plants and/or when measurements were taken on cloudy days. Hence high solar radiation and high atmospheric evaporative demand existing during summer period could probably be the major contributoiy factors for this reduction.