The binding levels and opiate receptor binding parameters were determined for 3H-naloxone in rat brain in the presence of NaCl added in vitro. An addition of NaCl at concentrations of 5-35 mM to the reaction medium caused an increase in the level of the antagonist receptor binding. The maximal level of 3H-naloxone reception activation was observed in the presence of 10-20 mM NaCl and was, on the average, 25%. Both the increase in the NaCl dose in vitro and its decrease caused a gradual diminution of the Na+ effect. An analysis of opiate receptor saturation with 3H-naloxone revealed that the label interacted with one type of the binding sites irrespective of NaCl concentration. The affinity of receptor binding sites for 3H-naloxone increased already at NaCl concentration of 2.5 mM. In contrast, the apparent maximal number of binding sites did not change after NaCl addition at concentrations which coincided with the intracellular Na+ level but was decreased with an increase (up to 50-100 mM) in NaCl present in the reaction mixture. The results obtained point to the existence of two different binding sites that are coupled with the 3H-naloxone reactive opiate receptor.