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1. The membrane potential of giant muscle fibres of Maia squinado was measured with an intracellular wire electrode. On applying L-glutamate to the fibre the cell deplorized and fluctuations of the membrane potential around its mean level--glutamate noise--were seen. 2. The variance of the glutamate voltage noise is proportional to the mean level of depolarization. The noise can be regarded as being caused by numerous exponentially decaying elementary voltage events about 5 X 10(-10) V in amplitude. The miniature excitatory junctional potential (min.e.j.p.) is approximately 6000 times the amplitude of the elementary voltage event produced by L-glutamate. 3. The power spectrum of glutamate voltage noise is a Lorentzian with a half-power frequency of approximately 20 Hz. 4. Min. e.j.p.s. decay exponentially with a time constant that coincides with the average lifetime of the elementary glutamate voltage event. 5. When glutamate is applied locally to a spot where extracellular min. e.j.p.s. can be recorded with a focal glass pipette, extracellular glutamate noise is seen. Glutamate noise could not be detected from elsewhere on the fibre. 6. The variance of the extracellular noise is proportional to the mean extracellular potential, and its power spectrum is a Lorentzian with a half-power frequency of about 110 Hz. 7. The extracellular min. e.j.p.s decay exponentially with a time constant that coincides with average lifetime of the elementary glutamate current event. 8. It is suggested that the decay of the quantal currents flowing at the excitatory junction is limited by the closure of the conductance channels in the post-synaptic membrane and not by the relaxation of the transmitter concentration in the synaptic cleft.