The role of nonionic diffusion in the transport of salicylic acid across rabbit proximal S2 and S3 segments was investigated using the in vitro isolated perfused tubule technique. The [14C] salicylic acid apparent reabsorptive permeability (P'I-b, 10(-5) cm/s) was measured at 19 degrees C with luminal solutions kept at different pH and bath maintained at pH 7.4. In S2 tubules, P'I-b was 25.0 +/- 3.5 when luminal pH was 6.0; P'I-b decreased to 8.1 +/- 1.4 and to 4.4 +/- 1.2 at a luminal pH of 6.5 and 7.0, respectively. In S3 tubules, P'I-b was 17.6 +/- 2.4, 5.3 +/- 1.1 and 3.4 +/- 1.1 at a luminal pH of 6.0, 6.5 and 7.0, respectively. There was a close correlation between P'I-b and the calculated proportion of nonionized salicylic acid present at each pH, indicating that only the nonionized molecule could diffuse in our conditions. We calculated the apparent permeability of nonionic salicylic acid and found 0.248 +/- 0.032 cm/s for S2 and 0.176 +/- 0.022 cm/s for S3 tubules. These calculated permeabilities were independent of pH.