The thermostimulated luminescence method was applied to study defective states in crystals of nucleic acid bases of various purity degrees, irradiated with the UV light within a range of 5-300 K or with γ-rays at 77 and 300 K. Carrier traps have been found in pure crystals. Comparing thermoluminescence curves for crystals of cytosine and its methyl derivatives and using results of quantum-mechanical calculations of ionization potentials of tautomeric forms of cytosine molecules, a conclusion has been drawn that some tautomers are present in the crystals studied. Defects resulting from interaction of cytosine molecules with water radiolysis products have been found in cytosine monohydrale crystals irradiated with γ-rays.