Two experiments were performed during the summer of 1981 to determine the effects of simulated acidic rain on seed yields of soybeans grown using standard agronomic practices. In one experiment, plants were shielded from all ambient rainfall and exposed to simulated rainfall in quantities equal to the average amount of rainfall that occurs at the site. Seed yields of soybeans exposed twice weekly to simulated rain of pH 4.1, 3.3, and 2.7 were, respectively, 10.7, 16.8, and 22.9% below yields of plants exposed to simulated rain of pH 5.6. A treatment-response function of seed yield vs rainfall pH was y=7.40 + 1.025x and had a correlation coefficient of 0.997 (y is seed mass per plant and x is the pH of the simulated rain). In a second experiment, soybean plants were not shielded from ambient rainfall (weighted mean hydrogen ion concentration equal to pH 4.04) and received only small volumes of simulated rainfall three times weekly. (Copyright (c) 1983 The New Phytologist.)