In pigs, behavioural responses were examined after administration of 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), a full agonist at 5-hydroxytryptamine (5-HT) receptors of the 5-HT1A subtype, and the pyrimidinylpiperazine derivatives ipsapirone and Bay Vq 7813 (2-[4-(2-pyrimidinyl)-1-piperazinylpropyl]-1,2-benzisothiazol++ +-3(2H) one-1,1-dioxide), which act as partial agonists at 5-HT1A receptors. The most prominent behavioural response examined after 8-OH-DPAT, 0.5 mg/kg i.m., ipsapirone, 2-5 mg/kg i.m., and Bay Vq 7813, 0.5-2 mg/kg i.m. or i.v., were head shakes. The potency of the three drugs to induce this behaviour correlated with their activity at 5-HT1A receptors as determined by inhibition of forskolin-stimulated adenylate cyclase, substantiating that the head shake response has potential as a quantitative probe of in vivo receptor function. The 5-HT2/5-HT1C receptor antagonist ritanserin did not counteract the head shakes induced by ipsapirone, suggesting that neither 5-HT2 nor 5-HT1C receptors are involved in mediation of this response to this 5-HT1A receptor agonist in pigs. Once daily administration of Bay Vq 7813 or ipsapirone for 3-5 days led to a reduction in the head shake response. 1-Pyrimidinylpiperazine (1-PP), a pharmacologically active metabolite shared by ipsapirone, Bay Vq 7813, and related pyrimidinylpiperazine derivatives, did not induce behavioural alterations in pigs. The data provide further evidence that marked species differences exist in functional responses to 5-HT receptor ligands.