Density functional theory (DFT) has been used to investigate the conformations and tautomeric forms of neutral tetracycline in aqueous solution. The results suggest that the extended conformation is 3-3.5 kcal mol(-1) more stable than the twisted one for equivalent tautomers and that as many as six different tautomeric forms lie within 10 kcal mol(-1) of the most stable one. The energetic preference for the extended conformation is a solvent effect. Calculated infrared, NMR, and UV/vis spectra are used to suggest ways of differentiating between tautomers. and conformations experimentally. It is shown that the C-13 chemical shifts of C4a, C5, and C6 can be used to distinguish between the twisted and extended conformations.