1. Single-channel currents were recorded from Xenopus oocytes which had been injected with complementary RNAs (cRNAs) for the neuronal nicotinic acetylcholine receptor subunits alpha 3 and beta 4. The co-expression of alpha 3 and beta 4 gave rise to three different channel types with conductances of 22, 18 and 13 pS. 2. The activity arising from the expression of these two subunits was compared with that observed when the alpha 3 subunit was co-expressed with an alternative beta-subunit, beta 2. The alpha 3 beta 4-receptors differed from alpha 3 beta 2-receptors in conductance, open times and most notably, in burst kinetics. The association of the beta 4-subunit with the alpha 3-subunit results in receptors which have a high probability of re-opening after closing, yielding protracted bursts of activity not observed when the alpha 3-subunit is associated with the beta 2-subunit. 3. All three alpha 3 beta 4-channel types had similar burst kinetics. However, the 13 pS conductance channels showed an additional long-lived open state to varying degrees when clusters of activity within an individual record were examined.