The crystal structure of the title compound, which is a formal analogue of 5-methyl- N 4-hydroxycytosine nucleosides, has been determined by X-ray diffraction. The space group is P2 1 c with a = 7.368 (2), b = 12.096 (3), c = 9.192 (4) A ̊ , β = 113.94 (3)°. Three-dimensional intensity data were collected with a four-circle diffractometer, and the structure was refined by block-diagonal least-squares to R = 0.053. The compound is in the imino form, and the exocyclic N 4-OH is located essentially in the plane of the pyrimidine ring, and syn to the ring N(3). There is an intramolecular hydrogen bond involving the N(3)-H as donor and O(4) as acceptor, viz. N(3)-H(31)···· O(4)-H. With this conformation, which probably prevails also in solution, the compound would be unable to participate in normal Watson-Crick base pairing. It is shown that a similar situation may prevail for N 4-hydroxycytosine nucleosides. The implications with regard to the molecular mechanism of hydroxylamine mutagenesis, with particular reference to the T-even bacteriophages, are discussed. Analogous considerations are applied to an examination of the possible behaviour of hydroxylamine-modified adenine nucleosides.