The potency of the serotonin 1A (5-HT1A) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT), to induce forepaw treading was increased 20-fold after co-treatment with the 5-HT2 agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). DOI induced head twitches which were inhibited by 8-OHDPAT. The putative 5-HT1B agonist, 1-(3-trifluoromethylphenyl)piperazine (TFMPP), had a weak effect on the responses to DOI or 8-OHDPAT. The forepaw treading induced by 8-OHDPAT plus DOI was inhibited by high doses of (-)-alprenolol, ketanserin or ritanserin, but was not influenced by the beta-adrenoceptor antagonist, ICI 118.551, or the 5-HT3 antagonist, ICS 205-930. A non-effective dose of (-)-alprenolol increased the inhibitory effect of ketanserin and ritanserin. These results indicate a complex and different interaction between 5-HT1A and 5-HT2 receptors in the expression of two behavioural responses mediated by 5-HT.