Kappa opioid agonists are known to inhibit dopamine release. We sought to determine the site of this action and the relationship of tolerance to this effect. Microdialysis perfusion of the nucleus accumbens in unanesthetized rats was used to monitor dopamine release, as well as DOPAC, HVA, and 5-HIAA efflux. Administration of the kappa agonist U-50488H (0.5-10 mg/kg, s.c.) resulted in a dose-related inhibition of basal dopamine release and a delayed reduction in HVA efflux. When added directly to the perfusion medium, U-50488H (10 microM) similarly reduced dopamine release and HVA efflux; however, a much higher concentration (1 mM) produced a transient increase in dopamine release. The more potent kappa agonist, spiradoline mesylate (5 mg/kg, s.c.) caused a more profound and long lasting reduction in dopamine release than that observed with U-50488H. Repeated injections of spiradoline (7 injections over 3 days at 5 and 10 mg/kg, s.c.) resulted in a persistent reduction in dopamine release with no further reduction in release being observed following an acute injection of spiradoline (1 mg/kg, s.c.). We conclude that kappa agonists act to inhibit dopamine release from the nucleus accumbens via a direct effect in that region, and that tolerance does not occur to this neurochemical effect. Thus, kappa agonists may prove useful in chronic conditions resulting from excessive dopamine release.