A naive dimensional reduction of the N=1, D=10 supergravity theory tha
t naturally arises in five-brane models is used to determine the role
of two fields which are basic ingredients of string models: the dilato
n and, among the moduli, the breathing mode. It is shown that, under t
he duality transformation that relates five-branes and strings, these
two fields exchange the roles of the 10-dimensional dilaton and the ra
dius of the compact manifold. A description of this phenomenon in term
s of the linear multiplets of the 4-dimensional supergravity is also p
resented.