Ikkai&Ooi [Ikkai,T.&Ooi,T. (1966)Biochemistry5, 1551± 1560] made a thorough study of the e ect of pressure on G- and F-actins. However, all of the measurements in their study were made after the release of pressure. In the present experiment in situ observations were attempted by using eATP to obtain further detailed kinetic and thermodynamic information about the behaviour of actin under pressure.