It is generally accepted that formaldehyde (FA) is a toxic substance. It is formed in various demethylation and transmethylation reactions. And there is an intracellular system for its detoxification. Meanwhile, its effect on the cell causes a different response: apoptosis, proliferation, differentiation and depends on the concentration of FA. Evolutionarily, FA is a component of the one-carbon transport system. Oxidation of the methyl group in the folate cycle is a proton donor for the reduction of NADP+. In the cell, FA spontaneously non-enzymatically binds to pterin, arginine, glutathione, which determines its participation in the metabolism of purines, nitric oxide and polyamines, and in the redox system. This review describes a number of metabolic pathways that are linked into a single system thanks to FA. FA is the main source of formate in the cell and closely interacts with arginine metabolic pathways, influencing the synthesis of nitric oxide and polyamines. The possible role of formaldehyde dehydrogenase (ADH5) in the regulation of intracellular pH is also considered.