A Zr-91 chemical shift of -1500 ppm (relative to (C5H5)(2)ZrBr2) is predicted for Zr@C-28 at the GIAO (gauge-including atomic orbitals)-B3LYP level using a medium-sized basis set and SCF optimized geometries. This value is even more shielded than the one predicted for hypothetical (eta(5)-C5H5)(4)Zr ca. -1100 ppm at the same level. A noticeable deshielding with repect to Zr@C-28 is indicated for Zr@C28H2, a model for exohedrally substituted derivatives. Electron correlation effects on delta(Zr-91), as assessed by GIAO-B3LYP vs GIAO-SCF results, are much larger for these endohedral Zr fullerenes than for any other Zr compound studied so far. According to the computed electric field gradients, quadrupolar line broadening should be large for the known (eta(5)-C5H5)(3)(eta(1)-C5H5)Zr but should be small for Zr@C28H2. Since Zr@C-28 has been detected mass- spectroscopically "in substantial yield" (Guo, T.; et al. Science 1992, 257, 1661), Zr-91 NMR spectroscopy is suggested as the analytical method of choice for Zr@C-28 and derivatives thereof.