One-loop determinant of Dirac operator in non-renormalizable models

We use proper-time regularizations to define the one-loop fermion determinant in the form suggested by Gasser and Leutwyler some years ago. We show how to obtain the polynomial by which this definition of ln detD needs to be modified in order to arrive at the fermion determinant whose modulus is invariant under chiral transformations. As an example it is shown how the fundamental symmetries associated with the NJL model are preserved in a consistent way.