The gauge including atomic orbital (GIAO) method at the self-consistent field (SCF) level of theory was used to calculate magnetic shielding tensors in free base porphyrin (H2P) and in magnesium (MgP) and zinc (ZnP) metalloporphyrins, as well as in the MgP(H2O) and MgP(H2O)(2) complexes. All principal components of the calculated shielding tensors are reported. We have recorded new solution NMR spectra for all three molecules under identical conditions, to ensure the reliability of small differences in their NMR shifts. The calculated isotropic values are compared with these values and with low-temperature solid-state shifts, which resolve the tautomerism in H2P. Agreement with experiment is good. It is encouraging that the calculations correctly reproduce most of the the small differences in the chemical shifts. A comparison of the observed and calculated NMR shifts of MgP leads to the conclusion that the species observed in tetrahydrofuran solution is either MgP(H2O)(2) or MgP(THF)(2). We have calculated the effect of the nut-of-plane displacement of the metal ion in metalloporphyrins, with a view to using NMR as a probe of nonplanarity. Only the N-15 shifts show any sensitivity.