Abstract Normal movements of the mammalian brain, caused by the arterial and venous pressure fluctuations of each cardiac and respiratory cycle, have made obtaining stable intracellular recordings from neurons difficult. This study quantitated the movements of the cats' brainstem and examined the effects of traditional neurophysiological techniques used to reduce pulsation. Two components of brain movement were recorded: (1) an arterial component—relatively low amplitude (110–266 μm) and short duration (330–400 ms) excursions corresponding to the pressure wave of each cardiac systole [A-wave]; and (2) a pulmonary component—slower (10–12/min), high amplitude plateau-like displacement (300–950 μm) lasting for a time (2.4–5.1 s) corresponding to the inspiration of each respiratory cycle [P-wave]. Pneumothoraces and mechanical ventilation combined with elevating the animal's head reduced the pulmonary component by an average of 68% and the arterial component by 40%. Cerebrospinal fluid drainage could reduce the P-wave component of movement by as much as 50%. To reduce arterial pulsations below 100 μm, the mean arterial pressure (MAP) had to be lowered to less than 40 mm Hg, which was not compatible with maintaining normal brainstem auditory evoked responses. Residual movements at MAPs greater than 50 mm Hg were still sufficient to make stable intracellular penetration of small neurons difficult. The authors suggest the solution to this problem is the development of a cardiopulmonary bypass system which generates a non-pulsatile flow of oxygenated blood, described in a companion paper.