Two-photon-exchange effect in radiative corrections to. pi. /sup 0/. --> gamma. e/sup -/e/sup +/

We present the results of a direct calculation of the two-photon-exchange contribution to the rare decay ..pi../sup 0/..--> gamma..e/sup -/e/sup +/. Contrary to previous expectations, we find this contribution to be non-negligible with important consequences for the measurement of the form-factor slope.