The ovarian histology of fathead minnows (Pimephales promelas) chronically exposed to three levels of environmental pH was examined for evidence of reproductive impairment. Exposures occurred in three experimental running water channels receiving Mississippi River water. One of these channels was not acidified and two were dosed with H2SO4. The fish exhibited ovarian histological changes and depression of reproductive success which were directly associated with the level of environmental stress experienced. The association between these three factors was most consistent and pronounced if the fish were sampled near the end of the spawning season.