Semiempirical AMI, PM3 and all electron MP2 and DFT nonempirical calculations for the normal and rare tautomeric forms of pyrimidine bases and their photohydrates were performed. It is predicted that the trans conformation of the imino form of cytosine photohydrate is more stable than its amino form in the gas phase. The Monte Carlo simulations showed that cytosine amino form is stabilized by a water more than the imino tautomer, while cytosine photohydrate in imino form is more stable than the corresponding amino form. The present results suggest that tautomerism is a possible mechanism of UV-light mutagenic effect.