We have demonstrated the slowing of a beam of cold (180 K) metastable neon atoms in a diffuse reflecting cylindrical cavity with monochromatic light. Using a well-collimated atomic beam entering the cavity, it is shown that stimulated emission has a major effect on the transverse velocity distribution. A large saturation parameter s results in a loss mechanism for the center line beam intensity. A Monte Carlo simulation shows that the light intensity should be evenly distributed with moderate saturation conditions, to optimize the slowing effect while minimizing the transverse diffusion.