Photoacoustic spectroscopy has been used to monitor the photosynthetic activity of the leaves of sugar maple saplings treated by watering the soil with simulated acid rain at different pH levels (2.S, 3.0, 3.5, 4.0, 4.5, 5.0 and 5.6). We also measured the relative growth rate of the plants and the pigment content of leaves. The results indicated an ambivalent effect of acidity: there was a linear decline (r = 0.92) in the photosynthetic O 2 evolution as the pH of the simulated rain was lowered from 5.6 to 3.0 and a jump when it reached 25, with a stimulation effect on the relative growth rate at lower pH levels. The photochemical energy storage and the relative growth rate increased from pH 5.6 to pH 4.0-4.5, then decreased at pH 3.0 and increased again at pH 2.5. The pattern of variation of the photochemical energy storage was fairly well correlated with the pigment content of the leaves (r = 0.83). It was concluded that acid rain preferentially affects photosystem II (PSII), inhibiting O 2 evolution, and less PSI. The difference between O 2 evolution and photochemical energy storage was explained in connection with cyclic electron transport around photosystem I (PSI).