The selective functions of D 3 receptors in the brain are still poorly understood, mainly because all the ligands active at dopamine D 3 receptors have also a high affinity for the D 2 receptors. However, it is possible to study selectively D 3 receptor function because some brain structures, such as the islands of Calleja, contain D 3 and not D 2 receptors. The position of the island of Calleja Magna in the rat brain makes it possible to inject dopamine D 3 ligands into the vicinity of these D 3 receptors, and to study their behavioral role, with no concomitant action on D 2 receptors. We studied the effects on body temperature and on locomotion of unilateral microinjections of D 2 /D 3 receptors ligands into the island of Calleja Magna and into the adjacent nucleus accumbens. The results show that D 3 agonists injected into the island of Calleja Magna decrease body temperature and that this effect is potentiated by simultaneous injection of the D 1 agonist SKF 38393. D 3 agonists have no effect on locomotor activity in the island of Calleja Magna. In the nucleus accumbens, the D 3 agonists have only weak effects on body temperature, but, when associated with a D 1 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 D 3 receptors are localized postsynaptically in the island of Calleja Magna.