This study was concerned with the effects of NaCl administered in vivo or added in vitro to isolated nuclei on [³H]tryptophan binding to rat hepatic nuclei assayed in vitro. Hypertonic (10.7%) NaCl administered in vivo to rats caused at 10 min a marked decrease in in vitro binding (total and specific) of [³H]tryptophan to hepatic nuclei. In vitro incubation of isolated hepatic nuclei, but not of isolated nuclear envelopes, with added NaCl (particularly at 0.125 × 10⁴ M and 0.25 × 10⁴ M) revealed significant inhibition of [³H]tryptophan binding. However, isolated hepatic nuclear envelopes prepared after in vitro incubation of isolated nuclei with added NaCl did show inhibition of [³H]tryptophan binding (total and specific) compared with controls. Other salts (KCl, MgCl₂, NaHCO₃, NaC₂H₃O₂, NaF, or Na₂SO₄), at similar concentrations to that of NaCl except for MgCl₂, when added to isolated nuclei did not appreciably inhibit nuclear tryptophan binding. Kinetic studies of in vitro nuclear [³H]tryptophan binding in the presence of 0.125 × 10⁴ M NaCl revealed that binding decreased at 0.5 h and continued to 2 h compared with nuclear [³H]tryptophan binding with controls (without NaCl addition). The results obtained in vivo in rats and those obtained in vitro with isolated hepatic nuclei revealed NaCl-induced inhibitory effects on [³H]tryptophan binding to hepatic nuclei. Although the inhibitory effects were similar under the two different experimental conditions, the mechanism for each may be different in that the NaCl concentration in hepatic cells after administration of NaCl in vivo was appreciably higher than the low levels added in vitro to the isolated hepatic nuclei.