Since the discovery of enzymic fermentation by Louis Paster, the idea had been widely spread in biochemistry that all molecular interconversions and interactions in living systems are provided by the enzymes. However, recent advances in sensitivity and accuracy of analytical methods lead to isolation and identification of a variety of products that cannot be mapped to a specific biochemical pathway. These products appear in the organism because the chemical characteristics of biomolecules are not limited to the needs of living systems realized through enzyme-catalyzed processes. This review for the first time attempts to summarize such nonenzymic interactions: (i) products of biogenic amine adduct formation to carbonyl-containing compounds in Pictet-Spengler reaction (endogenous neurotoxins which alter mitochondrial functions thus evidently participating in development of age-related cerebral aberrations, i.e., parkinsonism); (ii) products of Schiff base formation with subsequent Amadori rearrangement (nonenzymic glycation of proteins involved in age-related diabetic disorders and atherosclerosis); (iii) products of Michael addition of methylglyoxal, 4-hydroxynonenal, and other products of nonenzymic conversions of carbohydrates and lipids; (iv) products of biomolecule modification with nitrogen- and oxygen-derived free radicals which contribute to cancer and aging. Some applications of these problems in evolutionary and medical biology are reviewed.