The results by using ANS fluorescent probe and spin labels 5-NS show that the fluidity of L.(H~+-ATPase)+Mg~(2+) (H~+-ATPase from pig heart mitochondria reconstituted in the presence of Mg~(2+)) is less than that of L (H~+-ATPase)-Mg~(2+) (proteoliposome reconstituted in the absence of Mg~(2+)). But no significant difference in fluidity has been observed when both reconstituted systems were monitored by using spin labels 12-NS and 16-NS. This indicates that Mg~(2+) may cause changes in fluidity of the lipid molecules near the surfaces of the bilayers, but does not affect significantly the fluidity of the deeper layer of the reconstituted system.It is tentatively supposed that in the presence of Mg~(2+), enhancement of activities of reconstituted H~+-ATPase may be due to the Mg~(2+)-mediated change in physical state of the lipids in the more superficial region of lipid bilayers so as to ensure a suitable conformation of ATPase complex, thereby possessing higher activity. The results by using ANS fluorescent probe and spin labels 5-NS show that the fluidity of L. (H ~ + -ATPase) + Mg ~ (2+) (H ~ + -ATPase from pig heart mitochondria reconstituted in the presence of Mg ~ (2+) is less than that of L (H ~ + -ATPase) -Mg ~ (2+) (reconolosome reconstituted in the absence of Mg ~ (2+)). But no significant difference in fluidity has been observed when both reconstituted systems were monitored by using spin labels 12-NS and 16-NS. This indicates that Mg ~ (2+) may cause changes in fluidity of the lipid molecules near the surfaces of the bilayers, but does not affect significantly the fluidity of the deeper layer of the reconstituted system. It is tentatively supposed that in the presence of Mg ~ (2+), enhancement of activities of reconstituted H ~ + -ATPase may be due to the Mg ~ (2 +) - mediated change in physical state of the lipids in the more superficial region of lipid bilayers so as to ensure a suitable conformation of ATPase complex, thereby possessing higher activity.