Miniature excitatory post-synaptic potentials (MEPSPs) were recorded extra- and intracellularly from the coxal depressor muscle of the cockroach, Periplaneta americana. 2. Statistical analysis of the time intervals between MEPSPs showed that the variance-to-mean curve for the extracellular data sets lay significantly above the line predicted for a Poisson process. A shuffling procedure, however, made the series almost random. 3. The serial correlation coefficients for the extracellular data sets exceeded the confidence limits for various lags and most of them were positive. In comparison with the intensity function, the departure from the random process appeared to be mainly due to large positive correlation of intervals. 4. It is concluded that in insect neuromuscular junctions, the spontaneous release of transmitter at individual sites is more clustered than that predicted by a Poisson process.
