The excitor nerve which innervates the opener muscle of the crayfish walking leg was stimulated repetitively for prolonged periods of time. 2. After an initial facilitation and subsequent depression of the postsynaptic potentials, occasional stimulus pulses failed to evoke a postsynaptic response; eventually, every second stimulus pulse failed to produce a postsynaptic potential change. This is termed the 1:1 block state. 3. The block resulted from failure of the appropriate nerve action potentials to propagate through an identifiable region of the axon. This region was in the joint between the propodite and the carpopodite and was characterized by dense surrounding connective tissue. 4. Associated with block development is a decrease in axon conduction velocity and an increase in the amplitude of the postsynaptic potentials; this latter phenomenon is termed anomalous facilitation and results from a decrease in the extent of synaptic depression. 5. The cause of the conduction block resides in the associated nonneural tissue and not in some geometrical property of the axon. 6. It is concluded that extracellular accumulation of K+, because of limited diffusion possibilities and possibly reduced Schwann cell tissue, depolarizes the axon, leading to propagation failure.
