1. ABT-418 appeared to function as a relatively broad spectrum activator of neuronal nicotinic receptors, expressed in Xenopus oocytes, with little cross reactivity to the mammalian muscle receptor subtype. However, the relative potencies of ABT-418 at the various subtypes differed from those acetylcholine (ACh). For example, ACh was most potent at alpha 3 beta 2 (EC50 approximately 30 microM) and least potent at alpha 2 beta 2 (EC50 approximately 500 microM). ABT-418 was most potent at alpha 4 beta 2 and alpha 2 beta 2 (EC50 approximately 6 microM and 11 microM, respectively) and least potent at alpha 3 beta 4 (EC50 approximately 188 microM). 2. In addition to activating neuronal receptors, ABT-418 exhibited complex properties, including the inhibition of ACh responses. 3. The current responses elicited by relatively high concentrations of ABT-418 on the alpha 4 beta 2 receptor subtype were protracted beyond the application interval. The coapplication of ABT-418 with either of the use-dependent inhibitors bis(1,2,2,6,6-tetramethyl-4-pipendimyl)sebacate (BTMPS) or tetramethyl-pipenidine (TMP) eliminated the late protracted phase of the currents with only small effects on the initial activation phase. When the reversible inhibitor TMP was washed from the bath, the previously inhibited late current reappeared, suggesting that the observed mixed agonist-antagonist effects of ABT-418 and (+/-)-epibatidine on alpha 4 beta 2 were due to a concentration-dependent noncompetitive inhibition, an effect similar to that obtained for (-)-nicotine. 4. The inhibition of alpha 4 beta 2 receptors by ABT-418 was voltage-dependent. When high concentrations of ABT-418 were applied under depolarizing conditions, additional late currents could be observed under conditions which suggested that a build up of ABT-418 in an unstirred layer over the surface of the oocyte was occurring. This may have been due to the dissociation of the drug from channel blocking sites on the receptors themselves, or alternatively, from the plasma membrane of the cells.