I have analytically calculated the evolution of the three components of velocity dispersion of the stars in a galactic disk when these are scattered by massive gas clouds, in a generalization of Spitzer & Schwarzschild's (1953) calculation. The principal assumptions made are: (i) The stellar orbits obey the epicyclic approximation. (ii) The gas clouds are massive, long-lived and on circular orbits. (iii) The typical star-cloud encounter time is short compared to the orbital time. (iv) The evolution due to encounters is treated as a diffusion process.