We investigated the antinociceptive activity of Lysine Acetylsalicylate (LAS) after intracerebroventricular (icv) injection in experimental animals. The effect on tonic pain was studied by means of the Formalin test on 140 male Swiss mice. In a first group of animals icv LAS was injected at different doses (0.25-0.5-1 mg in saline solution 5 microliters). A second group received icv morphine 1 microgram in 5 microliters saline, and finally a third control group received icv 5 microliters saline. The effect of the compounds on the Formalin test was evaluated under blind conditions. Icv LAS had no effect on the nociceptive behaviour at doses of 0.25 and 0.5 mg, while a reduction of the licking time was evident after the injection of 1 mg of the drug. The time course and the degree of the analgesic effect of icv LAS was investigated and compared to the effect of icv morphine.