Malonaldehyde was reacted with cytidine in buffered aqueous solutions in the presence of acetaldehyde or formaldehyde. The reaction mixtures were analyzed by HPLC, and the products were isolated by preparative C18 chromatography and structurally characterized by UV absorbance, fluorescence emission, (1)H and (13)C NMR spectroscopy, and mass spectrometry. The major adducts formed in the reaction of malonaldehyde and acetaldehyde were identified as 7-(beta-D-ribofuranosyl)-4-methyl-6-oxo-6,7-dihydro-4H-pyrimido[1,6-a]pyrimidine-3-carbaldehyde (M(1)AA-Cyd) and 1-(beta-D-ribofuranosyl)-4-(3,5-diformyl-4-methyl-1,4-dihydro-1-pyridyl)pyrimidine (M(2)AA-Cyd). In the reaction of malonaldehyde and formaldehyde, the major product was identified as 7-(beta-D-ribofuranosyl)-6-oxo-6,7-dihydro-4H-pyrimido[1,6-a]pyrimidine-3-carbaldehyde (M(1)FA-Cyd). The highest yields of M(1)AA-Cyd and M(2)AA-Cyd, 3.2 and 0.5 mol %, respectively, were obtained in the reaction performed at pH 4.6 and 37 degrees C for 8 days, while M(1)FA-Cyd was produced at a yield of 0.3 mol % after 3 days of reaction at pH 4.0 and 37 degrees C. The products consist of units derived from malonaldehyde and acetaldehyde (M(1)AA-Cyd and M(2)AA-Cyd) or from malonaldehyde and formaldehyde (M(1)FA-Cyd), and are thus further examples of nucleoside modifications containing structural elements derived from aldehyde condensation reactions. Trace amounts of the adducts may be formed at physiological conditions and may be involved in the mutagenicity of the studied aldehydes.