Simulated acid rain has been reported to cause physiological changes in various plant species. Studies were conducted in 1983, 1984, and 1985 to determine the effect, of acid rain on physiological parameters in two soybean [Glycine max (L.) Merr.] cultivars. Simulated acid rain of pH 3.0, 4.2 and 5.6 was applied throughout the growing season to plots protected from ambient rain and grown on a Flanagan silt loam (fine, montmorillonitic, mesic Aaquic Argiudoll). Individual plants were evaluated for CO2 fixation, leaf water potentials, and leaf chlorophyll content. Microplots were evaluated for flowering and podding patterns, and dry matter and N partitioning. Rain pH did not affect CO2 fixation, water potentials, chlorophyll content, seed yield, shoot dry weight, or N accumulation. At pH 4.2 Amsoy 71 produced fewer flowers and pods than at pH 3.0 or 5.6. Abscised leaf dry weight of Amsoy 71 was greater at pH 3.0 and 4.2 than at pH 5.6 pH effects were detected for N partitioning, but these responses were dependent on year and cultivar. No physiological basis for reported soybean yield responses to acid rain of pH 3.0 was identified.