A variety of methods has been employed for the purpose of permitting direct observation of pial vessels (1, 2, 3, 4, 5). Another method, differing in various ways from those used previously, is described in this paper. This method has been developed in monkeys, is a simple one, and does no apparent harm as far as the health of the animal is concerned. Although the efforts of the authors to date have been limited to the attempt to observe changes in cerebral vessels, it is felt that the plastic insertion may lend itself to other purposes, such as the installation of electrodes, thermocouples, or ventricular needles.