The velocity space distortion of the ion distribution functions in a neutral injection heated tokamak plasma is investigated. First the solution of the time dependent Fokker-Planck equations describing the fast ion slowing down process and the collisional response of the background plasma ions is given. Then the form of the distortion of the background ions due to a stationary isotropic beam ion distribution within the plasma is presented. Using the time dependent solutions it is shown that during the initial phase of the injection the relaxation of the fast ion anisotropy is accompanied by the development of a high energy anisotropic tail on the plasma ion distribution functions. Finally the isotropic distortion in tokamaks operating close to the thermonuclear threshold (JET, TFTR, JT60), is calculated and the results indicate that in beam dominated plasmas significant distortion of the background ion distribution functions can occur.