  The total cross section for the pp to pp $\pi^0$ reaction at energies close
to threshold is calculated in a covariant one-boson-exchange model. The
amplitudes for the elementary BN to N$\pi^0$ processes are taken to be the sum
of s, u and t pole terms. The main contributions to the primary
productionamplitude is due to a sigma meson exchange. Both the scale and energy
dependence of the cross section are perfectly reproduced.
