1 The a5 subunit participates to the formation of native neuronal nicotinic receptors, particularly in autonomic ganglia. Like the related b3 subunit, a5 forms functional recombinant receptors if expressed together with a pair of typical a and b subunits, but its e ect on the properties of the resulting aba5 receptor depends on the a and b subunits chosen and on the expression system. We used a reporter mutation approach to test whether a5, like b3, is incorporated as a single copy in human a3b4a5 receptors expressed in oocytes. 2 As previously reported, the main indication of the presence of a5 in a3b4a5 wt was an increase in apparent receptor desensitization (compared with a3b4 receptors). If the a3b4a5 receptor bore a 9'T mutation in the second transmembrane domain of either a3 or b4, a5 incorporation produced a decrease in ACh sensitivity (by 4 fold for a3 LT b4a5 vs a3 LT b4 and by 40 fold for a3b4 LT a5 vs a3b4 LT ). The much greater e ect observed in a3b4 LT a5 receptors accords with the hypothesis that a5 takes the place of a b subunit in the receptor. 3 Introducing a 9'T mutation in a5 had no e ect on the agonist sensitivity of a3b4a5 receptors, but reduced apparent desensitisation, as judged by the sag in the current response to high agonist concentrations. 4 Introducing the 9'T mutation in a3 or b4 in the triplet receptor reduced the EC 50 for ACh by a similar extent (7 and 9 fold, respectively), suggesting that a3b4a5 receptors contain two copies each of a and b and therefore only one copy of a5.