G protein-activated inwardly rectifying K+ channels (GIRK, also known as Kir3) are activated by various G protein-coupled receptors. GIRK channels play an important role in the inhibitory regulation of neuronal excitability in most brain regions and the heart rate. Modulation of GIRK channel activity may affect many brain functions. Here, we report the inhibitory effects of various antidepressants: imipramine, desipramine, amitriptyline, nortriptyline, clomipramine, maprotiline, and citalopram, on GIRK channels. In Xenopus oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, the various antidepressants tested, except fluvoxamine, zimelidine, and bupropion, reversibly reduced inward currents through the basal GIRK activity at micromolar concentrations. The inhibitions were concentration-dependent with various degrees of potency and effectiveness, but voltage- and time-independent. In contrast, Kir1.1 and Kir2.1 channels in other Kir channel subfamilies were insensitive to all of the drugs. Furthermore, GIRK current responses activated by the cloned A1 adenosine receptor were similarly inhibited by the tricyclic antidepressant desipramine. The inhibitory effects of desipramine were not observed when desipramine was applied intracellularly, and were not affected by extracellular pH, which changed the proportion of the uncharged to protonated desipramine, suggesting its action from the extracellular side. The GIRK currents induced by ethanol were also attenuated in the presence of desipramine. Our results suggest that inhibition of GIRK channels by the tricyclic antidepressants and maprotiline may contribute to some of the therapeutic effects and adverse side effects, especially seizures and atrial arrhythmias in overdose, observed in clinical practice.