An implementation of analytic second derivatives for the coupled-cluster singles, doubles, and triples (CCSDT) model is reported and applied to the calculation of nuclear magnetic shielding constants of BH, HF, CO, N-2, N2O, and O-3. The CCSDT calculations yield further evidence for the high accuracy provided by chemical shift calculations employing the coupled-cluster singles, doubles (CCSD) approach augmented by a perturbative triples correction [CCSD(T)].