The ring-chain conformations of retinals within rhodopsin (Rh) and bacteriorhodopsin (bR) are estimated by means of ab initio NMR-shielding calculations, based on the localized orbital/local origin method. First, we calculated the C-13 shieldings for the unsaturated carbons of all-trans-retinal in double-xi level and confirmed that there isa good agreement between the, calculated and experimental data. Subsequently, the shielding calculations were carried out in the same. level for a retinal analogue to investigate the correlation between the C-13 chemical shieldings and-the C5=C6-C7=C8 dihedral angle (phi(6-7)). Characteristic magnetic properties are found in the C5 and C8 shieldings, which behave in periodic manners depending on the value of phi(6-7). The major source of the periodicity is attributed to the recovering and breaking of the conjugated system rather than to steric interaction because the sigma(22). and the sigma(33)-shielding tensor elements in the conjugated plane depend strongly on the phi(6-7) value, whereas the phi(6-7) dependence of the sigma(11) element perpendicular to the plane is less affected. We also propose that the C6-C7 conformation can be estimated through an observation of the chemical shift difference between C5 and C8 (Delta sigma(C8-C5)) without requiring the corresponding data for a reference compound. We conclude that only for the in-planar 6-s-trans form the calculated value of Delta sigma(C8-C5) has negative sign as observed experimentally and that the chromophore of bR takes the planar 6-s-trans form (phi(6-7) = similar to 180(degrees)). However, information only on the C5 and C8 shieldings is insufficient to determine whether the chromophore of Rh takes a skewed 6-s-cis or 6-s-trans form.