The depolarizing potency of the following three homological series: ‘BTM’: Me3N(CH2)nNMe3, n = 9–16; ‘D’: Me3NCH2CH2OCO(CH2)mCOOCH2CH2NMe3, m = 1–8; ‘D-mono’: Me3NCH2CH2OCO(CH2)mCOOCH2CH2H, m = 2–8, was compared by means of intracellular microelectrodes on identified giant neurons of the small parietal ganglion of Limnaea stagnalis after irreversible inhibition of cholinesterases. The equipotent molar ratio of the corresponding members of the series D : D-mono : BTM was approximately 1 : 3 : 10. These results indicate the important role of the ester groups in the depolarizing action of the quaternary and bisquaternary compounds as well as the importance of the second cationic head in series D. At variance with all other members of the D-series, suxamethonium (D-2) is not more active than decamethonium (BTM-10) and less potent than D-2-mono. The second cationic head reduces the depolarizing action of suxamethonium on the neurons studied. The study of some derivatives of the D-mono series had shown that replacing of the ‘second’ (more remote from the nitrogen) ester group by two methylene groups strongly reduces the depolarizing activity but the ‘reversal’ of the second ester group (that is the transposition of the ether oxygen and the carbonyl group) does not change the activity. The contribution of the second ester group in the interaction of the members of the D-mono series with the cholinoceptive membrane is discussed.