A fully automated instrument was developed that measured the amount and time-course of indentation of arterial intima by a spherical tipped probe 25 μm in diameter loaded routinely by forces of 10 μN. The relationship of depth of indentation to time was non-exponential but reached an apparent asymptotic value in about 6 s. Routine indentations were recorded 10 s after loading, the relationship of indentation and force being essentially linear between 0-100 μN. The instrument is an order of magnitude more sensitive than one previously described (Gow and Vaishnav, 1975, J. appl. Physiol. 38, 344-350) and produces indentations of 1-4 μm in the intima of rabbit thoracic aorta with a reproducibility of 1-2%. Preliminary usage of the new microindentor has shown increases in the compliance of the thoracic aorta intima in rabbits during cholesterol feeding.