The absorption of intense laser light is found to be reduced when targets are irradiated by 1.06-\ensuremath{\mu} m light with long pulse widths (150-400 ps) and large focal spots (100-250 \ensuremath{\mu}m). Estimates of Brillouin scatter which account for the finite heat capacity of the underdense plasma predict this reduction. Spectra of the back-reflected light show red shifts indicative of Brillouin scattering.