The effects of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on the behaviour of mice were studied. 8-OH-DPAT given i.v. in doses greater than 1 mg/kg induced the distinct 5-HT syndrome, including head weaving, hindlimb abduction, forepaw treading and tremor. The 8-OH-DPAT-induced behaviour was not affected by the 5-HT depleter, p-chlorophenylalanine. Reserpine, which depletes monoamines, significantly decreased the head weaving elicited by 8-OH-DPAT, although it did not reduce the other components of the behavioural syndrome. The non-specific 5-HT receptor antagonist, metergoline, attenuated the 8-OH-DPAT-induced behaviour, while the 5-HT2 receptor antagonist, ketanserin, was without effect. In addition, the 5-HT1A receptor antagonist, spiperone, inhibited the 5-HT syndrome elicited by 8-OH-DPAT, while the dopamine receptor antagonist, haloperidol, affected only the head weaving. These results suggest that 8-OH-DPAT-induced behaviour in mice is mediated by the postsynaptic 5-HT1A receptor.