Several assumptions underlying the use of intracellularly applied tetraethylammonium (TEA) for assessing monosynaptic connexions were evaluated in identified neurones of Aplysia. 2. In the R2 neurons, intrasomatic TEA application prolongs the duration of the intrasomatically recorded action potential. Subsequently, the action potential in the axon of R2, recorded extracellularly 4-7 mm from the soma, was also prolonged. 3. Intracellular application of TEA into the somata of the multi-action interneurone L10 enhances the duration of the L10 AP and results in larger and more prolonged post-synaptic potentials (p.s.p.s) recorded from neurones believed to be connected monosynaptically with L10. The action potential duration and wave form of p.s.p.s elicited by nerve stimulation in these same post-synaptic neurones were unaffected during the time L10-mediated p.s.p.s were potentiated. 4. Following TEA injection into L10 the p.s.p. recorded in neurone L7 changes wave form in a manner similar to that observed when L10 is tetanized. 5. It is concluded that TEA migrates from its intracellular site of application, does not leave the injected neurone in significant quantities, and alters the wave form of the p.s.p in only those neurones connected monosynaptically to the injected neurone.
