Piperidine derivatives of hemicholinium-3 were synthesized, which included the following spacing groups between cationic heads: trans-trans-bicyclohexyl, phenanthrene, naphthalene, and biphenyl. Relatively minor structural alterations in these series of compounds resulted in several different types of pharmacological actions related to cholinergic transmission. Structural requirements of the compounds are discussed and include internitrogen distance, structural planarity, spacing groups, and positional isomerism of the quaternary cationic heads. Selected quaternary piperidine derivatives with 14 A inter-atomic distance between the cationic heads exhibit potent HC-3 like activity which is enhanced if a molecule has a nonpolar space filling group (4-methyl piperidine) approximately 3.7 A from the corresponding quaternary cationic head. With selected piperidine ring substitutions, active tertiary amines were also identified. Compounds containing C = O in the spacing moiety were active inhibitors of cholinesterase with some derivatives being nearly as active as physostigmine. When the C = O moiety was reduced to -CH2 and a 2 or 3-CH3 piperidine ring was present, potent non-depolarizing, short acting neuromuscular blocking agents were obtained.