The influence of adenosine receptor ligands on ketamine-induced locomotor activity was studied in mice. Ketamine-induced hyperactivity (10 mg/kg) was significantly and dose-dependently attenuated by CGS 21680 (selective A(2A) receptor agonist), and NECA (A1/A2 adenosine receptor agonist), but not by CPA (cyclopentyladenosine, selective A1 adenosine receptor agonist). Motor activity produced by subthreshold dose (2.5 mg/kg) of ketamine was significantly increased by DMPX (selective A1 receptor antagonist) and caffeine (A1/A2 adenosine receptor antagonist), but not by DPCPX (selective A1 adenosine receptor antagonist). These results suggest that adenosinergic system is involved in ketamine-induced motor activity and seem to indicate a predominant role of A(2A) adenosine receptor in this effect.