4-hydroxynonenal is a major product of lipid peroxidation. It was firstly studied under the point of view of its toxicity, as it is an easily diffusable substance, thought to be able to explain the "far damages" seen in conditions of increased lipid peroxidation. Really, when used at concentration from 10 microM to 1 mM, usually referred to as high concentrations, the aldehyde is able to produce strong inhibitions of several enzymatic activities. When used, however, at concentration of 1 microM or lower, it displays a lot of activities regarding especially cell multiplication and differentiation. As the concentrations indicated above are usually found in normal tissues, these effects may be considered as physiological. As a low level of lipid peroxidation exists in normal tissues, the aldehyde displays signalling activities in normal cells. Among them, it is to consider the stimulation of neutrophil chemotaxis, the strong activation of plasmamembrane adenylate kinase, the strong activation of membrane phospholipase C, both in hepatocytes and neutrophils, the block in the expression of the oncogene c-myc in human leukemic cells, accompanied by differentiation of the same cells, the effects on the cyclins and the activity of E2F transcription factor, the strong increase of the expression of the gene for procollagen alfa1(I), occurring due to the activation of the c-jun/junkinases/AP-1 pathway. Moreover, it is able to block the activity of the PDGF-beta receptor. The last facts allow to think that a hydroxynonenal pathway works in the production of fibrosis.