A theoretical analysis is reported for the first triplet (T) to ground state (N) radiationless transition of the ethylene molecule. This work is based on ab initio calculations of the potential energy surfaces for the T and N states taking into account CC and CH bond stretching as well as HCH bending and torsion of the CH2 groups. The radiationless transition rates are calculated for different molecular vibrational levels using the Fermi-Wentzel golden rule formalism. The interaction matrix element [TV-intN] is approximated by the electronic spin-orbit coupling matrix element between the N and T states, integrated over the torsional coordinate; the effect of the three totally symmetric normal modes on the calculated rates is accounted for by their Franck-Condon factors. The transition rates calculated in this work are on the order of 10(6)-10(8) s(-1) if one or two levels of the totally symmetric modes are excited, which is in good agreement with the global values predicted earlier by Salem and Rowland.