Juvenile yellow perch (Perca flavescens), rock bass (Ambloplites rupestris), black crappie (Pomoxis nigromaculatus), and largemouth bass (Micropterus salmoides) exposed to acidified soft water ranging from pH 7.0 to 4.5, maintained blood osmotic homeostasis for up to 30 days. At pH 4.0, blood osmotic homeostasis was lost and all but the yellow perch died within 30 days. Blood osmolality decreased from a normal value of 280-320 mmol/kg to < or = 200 a few days before death. Yellow perch exposed to pH 4.1 were emaciated and near death within 58 days. Gill hyperplasma and chloride cell proliferation were observed in these species at pH 4.5 and 4.0. Field testing of these parameters as early warning indicators of acid stress is in progress at Little Rock Lake in northern Wisconsin. Little Rock Lake is a two-basin lake separated at the narrows by a plastic curtain. One basin is being acidified by the addition of H2504. Every two years the pH of the acidified basin is being reduced by 0.5 pH units. Examination of fish from basin at each pH interval is now being or will be conducted.