Title: HUT Observations of Time-Variable Absorption in NGC 4151

Abstract: We present the results of ultraviolet spectral monitoring of the Seyfert galaxy NGC 4151 performed by the Hopkins Ultraviolet Telescope during the Astro-2 mission. We show how the response of the Lyman continuum optical depth to variations in the continuum flux may be used to derive estimates of the distance of the absorbing gas from the central object. In this case, it seems that the absorbing gas is located $\leq 50$~pc away from the central source.