We have comparatively investigated the effects of Hardwickiic acid and Salvinorin A on the K(+)-evoked overflow of [(3)H]noradrenaline ([(3)H]NA) and [(3)H]dopamine ([(3)H]DA) from mouse hippocampal and striatal nerve terminals, respectively. The K(+)-evoked overflow of [(3)H]DA was inhibited in presence of Salvinorin A (100 nM) but not in presence of Hardwickiic acid (100 nM). Hardwickiic acid (100 nM) mimicked Salvinorin A (100 nM) in facilitating K(+)-evoked hippocampal [(3)H]NA overflow and the two compounds were almost equipotent. Facilitation of [(3)H]NA overflow caused by 100 nM Hardwickiic acid was prevented by the κ-opioid receptor (KOR) antagonist norbinaltorphimine (norBNI, 100 nM) and by the selective δ-opioid receptor (DOR) antagonist naltrindole (100 nM), but was not altered by 100 nM D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP), a selective μ- opioid receptor (MOR) antagonist. We conclude that Hardwickiic acid modulates hippocampal [(3)H]NA overflow evoked by a mild depolarizing stimulus by acting at presynaptic opioid receptor subtypes.