  We compute the contribution to the three--point temperature correlation
function of the Cosmic Microwave Background coming from the non--linear
evolution of Gaussian initial perturbations, as described by the Rees--Sciama
(or integrated Sachs--Wolfe) effect. By expressing the collapsed three--point
function in terms of multipole amplitudes, we are able to calculate its
expectation value for any power spectrum and for any experimental setting on
large angular scales. We also give an analytical expression for the {\it rms}
collapsed three--point function arising from the cosmic variance of a Gaussian
fluctuation field. In the case of {\it COBE} DMR, we find that the predicted
signal is about three orders of magnitude below that expected from the cosmic
variance.
