The selective functions of D3 receptors in the brain are still poorly understood, mainly because all the ligands active at dopamine D3 receptors have also a high affinity for the D2 receptors. However, it is possible to study selectively D3 receptor function because some brain structures, such as the islands of Calleja, contain D3 and not D2 receptors. The position of the island of Calleja Magna in the rat brain makes it possible to inject dopamine D3 ligands into the vicinity of these D3 receptors, and to study their behavioral role, with no concomitant action on D2 receptors. We studied the effects on body temperature and on locomotion of unilateral microinjections of D2/D3 receptors ligands into the island of Calleja Magna and into the adjacent nucleus accumbens. The results show that D3 agonists injected into the island of Calleja Magna decrease body temperature and that this effect is potentiated by simultaneous injection of the D1 agonist SKF 38393. D3 agonists have no effect on locomotor activity in the island of Calleja Magna. In the nucleus accumbens, the D3 agonists have only weak effects on body temperature, but, when associated with a D1 agonist, strongly stimulate locomotor activity. The effects on body temperature of unilateral microinjections of dopamine agonists into unilaterally dopamine-depleted animals are the same as those in nondepleted ones. This indicates that the D3 receptors are localized postsynaptically in the island of Calleja Magna.