Expression of serotonin (5-hydroxytryptamine; 5-HT), 5-HT receptors 1A (5-HT1AR) and 2A, and serotonin transporter protein (SERT) was studied in positive epicutaneous reactions to nickel sulphate in nickel-allergic patients, at 72 h post-challenge with the antigen. In addition, the effects of 5-HT2AR agonist 2,5-dimethoxy-4-iodoamphetamine (DOI), and the selective serotonin reuptake inhibitors (SSRIs) citalopram and fluoxetine, were tested on nickel-stimulated peripheral blood mononuclear cells from nickel-allergic patients, regarding their proliferation and interleukin (IL)-2 production, as well as the effect of these SSRIs on a murine Langerhans' cell-like line (XS52), regarding its IL-1beta production. Serotonin-positive platelets were increased in the inflamed skin compared with control skin. A decrease (p <0.01) in 5-HT1AR-positive mononuclear cells was evident in the eczematous skin compared with control skin, whereas 5-HT2AR- and SERT-positive cells were increased (p <0.001 for both) in the eczematous skin. Treatment of nickel-stimulated peripheral blood mononuclear cells with 5x10(-5) mol/l of DOI inhibited (p <0.01) the proliferation of nickel-stimulated peripheral blood mononuclear cells, while no effect was found regarding IL-2 production. Citalopram at 10(-6) mol/l tended to inhibit the production of IL-1beta by the XS52 cell line. These results indicate the implication of the serotonergic system in the contact allergic reaction.