1. It has been reported that at the frog neuromuscular junction at temperatures around 0 degrees C the release of transmitter quanta following nerve stimulation becomes disrupted, and the facilitation obtained after a second stimulus is no longer detectable. We thought that further investigation might give insight into the mechanism of quantal release, so we undertook experiments on Rana pipiens and Rana berlanieri. 2. In these species neuromuscular transmission occurs at temperatures as low as -0.8 degrees C. As the temperature is decreased further, transmission fails, apparently by a block in nerve conduction. The number of quanta released per stimulus decreases as temperature is lowered, with a Q10 of approximately 2.4. Owing to the decrease in the quantal output and the probabilistic nature of the release process, in occasional single records of an end-plate current (EPC), the pattern of release appeared disrupted. The kinetics of quantal release was studied by the use of a deconvolution method, which requires recording of EPCs and miniature EPCs (MEPCs) in preparations in high Mg(2+)-low Ca2+ solution. At approximately 0 degrees C the pattern of quantal release was similar to that at higher temperatures, although with a slower time course. At 0 degrees C the peak of release occurred approximately 3.5 ms after onset. 3. In our experiments there was almost no difference in the frequency of MEPCs at 22 degrees C and at 0 degree C. 4. We observed as much facilitation to a second stimulus at 0 degree C as at 10 degrees C. The Q10 for the decay of facilitation with time was between 1.9 and 2.3.(ABSTRACT TRUNCATED AT 250 WORDS)