1. The effects of tacrine (9-amino-1,2,3,4-tetrahydroacridine), velnacrine (HP029, 9-amino-1,2,3,4-tetrahydroacridin-1-ol maleate), suronacrine (HP128, 9-benzylamino-1,2,3,4-tetrahydroacridin-1-ol maleate), and 3,4-diaminopyridine on neuromuscular transmission were compared on isolated nerve-muscle preparations. 2. Tacrine, HP029, and 3,4-diaminopyridine augmented responses of chick biventer cervicis preparations to nerve stimulation, with tacrine and HP029 increasing responses to exogenously applied acetylcholine. HP128 blocked responses to nerve stimulation and to carbachol, but increased responses to acetylcholine. 3. In mouse diaphragm preparations that were partially paralysed by tubocurarine or low calcium solutions, tacrine, HP029, and 3,4-diaminopyridine reversed the twitch block. HP128 deepened the block. 4. In mouse triangularis sterni preparations, tacrine and HP029 prolonged the decay phase of endplate potentials and miniature endplate potentials, but had no effect on quantal content at 36 degrees C; above 10 microM, they reduced endplate potential amplitude. 3,4-Diaminopyridine increased quantal content without affecting the time course of the endplate potentials. HP128 (1-10 microM) had no effect on amplitude or time course of endplate potentials, but reduced their amplitude at higher concentrations. 5. Extracellular recording of nerve terminal currents from triangularis sterni preparations revealed that 3,4-diaminopyridine and HP128 had a selective blocking action on the waveform associated with K+ currents, tacrine reduced and prolonged the K(+)-related waveform, and HP029 had nonselective blocking actions only seen at high concentrations. 6. Tacrine and HP029 behave predominantly as anticholinesterase agents, while HP128 has weaker anticholinesterase actions that are masked by cholinoceptor blockade. Tacrine and HP128, but not HP029, have some blocking actions on K+ currents of mouse motor nerve terminals.