The influence of 4-hydroxynonenal (HNE), 4-hydroxytetradecenal (HTDE) and methylglyoxal (MGL) on rat polymorphonuclear leukocyte locomotion has been studied by means of boyden chambers. The two alkenals have been found to exert a chemotactic activity in a range of concentrations between 10-9 and 10-5 M, the most potent chemical being HTDE. The chemotactic effect, however, appeared influenced by the concentration of serum albumin in the medium. In addition, chemotactic doses of the same aldehydes were unable to affect the cell random migration. Higher concentrations of these alkenals depressed both the directed and random locomotion. Methylglyoxal appeared deprived of chemotactic power when assayed at the doses that were active with the above mentioned alkenals and it did not interfere with random migration. 5 X 10-4 M Methylglyoxal exerted a slight, but significant, inhibition of the locomotion events. These results suggest that aldehydes, present at the inflammatory sites, as final products of lipid peroxidation, might play a role in modulating the granulocyte cell locomotion, therefore interfering with the development of phlogosis.