Using proton NMR, the solution conformation of the A ring of vitamin D 3 and its analogs has been studied by application of the Karplus relation to the observed coupling constants. The A-ring conformations of vitamins D 3, D 2, and 25-hydroxyvitamin D 3 were found to be solvent dependent, with a clear preference for an equatorial hydroxyl group in polar solvents such as methanol and dimethyl sulfoxide. Conversion of the hydroxyl group to an acetate did not affect solution conformation appreciably, but the corresponding t-butyl-dimethylsilyl ether derivative of vitamin D 3 showed a strong preference for the 3β-equatorial conformer. The A-ring conformation of the active hormone, 1,25-dihydroxyvitamin D 3, which has two hydroxyl groups competing for the equatorial position, was found not to be solvent-dependent.