  We have measured the spin-forbidden decay rate from (3s3d)$^{1}$D$_{2}$ $\to$
(3s3p)$^{3}$P$_{2,1}$ in $^{24}$Mg atoms trapped in a magneto-optical trap. The
total decay rate, summing up both exit channels (3s3p)$^{3}$P$_{1}$ and
(3s3p)$^{3}$P$_{2}$, yields (196 $\pm$ 10) s$^{-1}$ in excellent agreement with
resent relativistic many-body calculations of [S.G. Porsev et al., Phys. Rev.
A. \textbf{64}, 012508 (2001)]. The characterization of this decay channel is
important as it may limit the performance of quantum optics experiments carried
out with this ladder system as well as two-photon cooling experiments currently
explored in several groups.
