  In this paper we extend the notion of rearrangement of nonnegative functions
to the setting of Carnot groups. We define rearrangement with respect to a
given family of anisotropic balls B_r or equivalently with respect to a gauge
|x|, and prove basic regularity properties of this construction. If u is a
bounded nonnegative real function with compact support, we denote by u* its
rearrangement. Then, the radial function u*. is of bounded variation. In
addition, if u is continuous then u* is continuous, and if U belongs to the
horizontal Sobolev space, we found a generalization of the inequality of Polya
and Szeg\"o.
