In this study, the involvement of serotonergic and dopaminergic receptors in the modulation of the head-twitch (HTW) response to the 5-hydroxytryptamine (5-HT)2A/5-HT2C agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, was characterized in rats using novel and selective ligands at 5-HT2A, 5-HT2C, D1, D2 and 5-HT1A receptors. HTW were dose-dependently inhibited by the 5-HT2A/2C antagonists, ritanserin, metergoline, mesulergine, mianserin, ICI 169,369 and LY 58,537, by the preferential 5-HT2A antagonist, ketanserin and by the novel, selective 5-HT2A antagonist, SR 46349B. A further selective 5-HT2A antagonist, MDL 100,907, very potently abolished HTW (ED50 = 0.005 mg/kg). The order of relative potency correlated highly with their affinity at 5-HT2A (r = 0.83) but not 5-HT2C receptors (r = 0.06). In addition, the novel, selective 5-HT2C antagonist, SB 200,646A, failed to abolish HTW and the 5-HT2C agonists/5-HT2A antagonists, 1-(3-chlorophenyl)piperazine and 1-(3-trifluoromethylphenyl)piperazine, blocked, rather than elicited, HTW. The D1 antagonists, SCH 23390, NNC 112, NNC 756, SCH 39166 and A 69024, in this order of relative potency that correlated with their affinity at D1 receptors (r = 0.98), blocked HTW. The D2 antagonists, raclopride, eticlopride and haloperidol also blocked HTW. The 5-HT1A agonists, S 14671, S 14506, 8-hydroxy-2-(di-n-propylamino)tetralin, buspirone, ipsapirone and (+)-flesinoxan, abolished HTW. The action of 8-hydroxy-2-(di-n-propylamino)tetralin was blocked by (-)-tertatolol (ID50 = 4.5 mg/kg), a novel 5-HT1A receptor antagonist. Similarly, (-)-tertatolol attenuated the action of S 14506 and abolished that of S 14671, buspirone and ipsapirone. A role of postsynaptic 5-HT1A receptors in the action of 5-HT1A agonists was suggested by the finding that parachlorophenylalanine (3 x 300 mg/kg, i.p.), which depleted cerebral pools of 5-HT, did not modify the activity of ipsapirone. The present data demonstrate that 5-HT2A receptors mediate HTW in rats and that both D1 and D2 receptors as well as (postsynaptic) 5-HT1A receptors play a role in their expression.