Bullfrog tadpoles (Rana catesbeiana) were pre-exposed (7d) to 5 different solutions which varied in ionic composition and pH. Uni-directional sodium influx was then measured over a range of sodium concentrations (50-2000 micrometers) and at 2 pH's (4.0 and 5.8). Tadpoles pre-treated in either distilled water or pH 4.0 soft water exhibited higher sodium influx than animals pre-treated in pH 5.8 soft water. Pre-exposure to solutions high in calcium or sodium reduced sodium influx. Tadpoles pre-treated in pH 4.0 soft water exhibited an increased V sub max for sodium transport, while K sub m was unchanged relative to tadpoles pre-treated in pH 5.8 soft water. In contrast, K sub m was increased and V sub max was unchanged in tadpoles pre-exposed to high concentrations of calcium or sodium. Within each pre-exposure treatment, influx was inhibited in pH 4.0 test water relative to pH 5.8 test water. However, the magnitude of inhibition was lessened with increasing external concentrations of sodium. (Copyright (c) Springer-Verlag 1986.)