Cuspidal representations of a reductive group are largely determined by their local components. In the function field case, it is important to describe how the number of cuspidal representations with given local conditions changes under base changes. Assuming Arthur's simple trace formula for function fields, this number should be given by the sum of Gross's L-functions attached to some reductive groups. In this talk, I will explain the expression of the sum of L-functions and its behavior under base changes for some classical groups.