The internalization of [H-3]propionyl[Met(O-2)(11)]SP(7-11) which binds one binding site and of [H-3][Pro(9)]SP which binds the two binding sites associated with the NK-1 receptor has been examined in CHO cells. The quantity of [H-3][Pro(9)]SP measured inside the cytoplasm in kinetic experiments is fully temperature-dependent. In contrast, [H-3]propionyl[Met(O-2)(11)]SP(7-11) internalization reaches the same extent whatever the temperature, although the rate slowed down with lower temperature. The extent of internalization of [H-3][Pro(9)]SP relative to the total specific bound is biphasic, when the extent of internalization of [H-3]propionyl[Met(O-2)(11)]SP(7-11) remains constant. For [H-3][Pro(9)]SP, a high-affinity high-yield component inhibited in the presence of propionyl[Met(O-2)(11)]SP(7-11) and a low-affinity low-yield component in the internalization process could be determined. Saturation studies show that [H-3][Pro(9)]SP-binding parameters are insensitive to both phenylarsine oxide and monensin treatment, whereas [H-3]propionyl[Met(O-2)(11)]SP(7-11) maximal binding is decreased in both cases. Altogether, these data suggest that the two radiolabeled peptides should not follow the same internalization pathway