The role of the endothelium in regulating transmural fluid filtration into the artery wall under pulsatile pressure, and the effects of changes in pulsatile frequency on filtration, have received little attention. Previous experiments (1) demonstrated significantly increased filtration after initial onset of pulsatile pressure compared to that predicted using parameters measured under steady pressure. In order to determine the role of the endothelium in this phenomenon, the following experiments were performed on five New Zealand White rabbits (anesthetized with 30 mg/kg pentobarbital sodium). One of each pair of carotid arteries was de-endothelialized and filtration measurements under steady and pulsatile pressure compared with those made in intact vessels (1). To determine the effect of increasing pulsatile frequency on arterial filtration, transmural filtration was measured using pulsatile pressure frequencies of 1 Hz, followed by 2 Hz, in another set of intact arteries (6 arteries, 3 animals). For de-endothelialized vessels, the initial increase in filtration after onset of pulsatility was similar to that observed in intact vessels, but the subsequent reduction in filtration was less abrupt. When pulsatile frequency was increased from 1 to 2 Hz in intact arteries, an initial increase in filtration was observed, similar to that obtained after onset of pulsatile pressure subsequent to a steady pressure. The observed responses of arteries to pulsatile pressure, with and without endothelium, or undergoing a frequency change, suggest a dynamic role for the endothelium in regulating transvascular transport in vivo.