1-(3-Chlorophenyl)piperazine (mCPP) (0.125-1.0 mg/kg i.p.), previously shown to inhibit social interaction, dose-dependently reduced exploration of the open arms of an elevated plus-maze. These findings suggest anxiogenic properties. The effect of mCPP was more potently inhibited by 1-(1-naphthyl)piperazine than by ketanserin, indicative of its mediation via activation of 5-HT2C rather than 5-HT2A receptors. The 5-HT1B receptor agonist CGS 12066B did not antagonise the anxiety-like response to mCPP, and further reduced exploration at the highest dose tested (10 mg/kg i.p.). Depletion of serotonin (5-HT) by p-chlorophenylalanine (PCPA, 150 mg/kg/day x 3) did not prevent the response, although PCPA itself increased open arm exploration. The 5-HT1A/B and beta-adrenoceptor antagonist 1-propanolol (5 mg/kg i.p.) and the peripheral beta 1-receptor antagonist atenolol (20 mg/kg i.p.) showed no significant activity on the plus-maze either alone or against the anxiogenic effect of mCPP. These results indicate that mCPP induces anxiety in the rat in the elevated plus-maze primarily by stimulation of postsynaptic 5-HT2C receptors, and suggest that sympathomimetic effects of mCPP are not involved.