Halothane-anaesthetized cats implanted with several push-pull cannulae were used to study the effects of the unilateral application of GABA (10(-5) M, 30 min) into thalamic intralaminar nuclei on the in vivo release of (3H)-dopamine [3H)-DA) newly synthesized from (3H)-tyrosine in both caudate nuclei (CN) and substantiae nigrae (SN). GABA applied into the left centralis lateralis nucleus (CL) elicited symmetric changes in the two CN, (3H)-DA release being initially reduced and thereafter markedly increased. On the contrary, an asymmetric pattern of responses was observed in the two CN during and after GABA application into the left centrum medianum parafascicular complex (CM/PF) since (3H)-DA release was decreased ipsilaterally and enhanced contralaterally. Changes in (3H)-DA release intervening in the two SN appeared to be triggered by processes independant from those operating in the CN. Hence, an immediate increase in (3H)-DA release with a time course distinct from that observed in other structures occurred in the contralateral SN after the application of GABA either into the left CL or the left CM/Pf. Furthermore in both cases, a biphasic response (decrease followed by an increase) similar to that intervening in the CN following GABA application into the CL was seen in the ipsilateral SN. Autoradiographic studies performed with (14C)-GABA (10(-5) M) revealed that the amino-acid did not spread out from the site of application. During and after GABA application (10(-5) M, 30 min) into the CL or the CM/Pf neuronal firing was markedly enhanced not only locally but also into respective contralateral homologous structures. These results further confirm the important and specific roles of distinct thalamic nuclei in the bilateral regulation of DA release from dendrites and nerve terminals of the nigro-striatal dopaminergic neurons.