Recordings of individual e.p.s.p.s evoked by the action of single medial gastrocnemius Ia fibres have been made from medial gastrocnemius motoneurones. In many motoneurones the action of two Ia fibres has been observed and the properties of the e.p.s.p.s compared. 2. For sixty-three pairs of averaged e.p.s.p.s, each from the same motoneurone, the ratio of half-widths was plotted against the ratio of rise times. These results were compared with theoretical values derived from the Rall compartmental model. It was found that variations in synaptic current time courses and differences in the termination of localized synaptic terminals were not sufficient to account for all the data. 3. Amplitude and rise time were inversely related but the correlation coefficient was very low. For pairs of e.p.s.p.s in the same motoneurone the e.p.s.p. with the fast rise time was larger than that with the slow rise time in forty-eight of sixty-three cases. 4. In a given motoneurone individual e.p.s.p.s evoked by the action of different Ia fibres did not vary greatly in amplitude. The ratio of peak amplitudes was less than 3 for 86% of the pairs of e.p.s.p.s examined, and the maximum was 4-8. 5. Amplitude histograms were constructed for individual e.p.s.p.s at thirty-three synapses. Twenty-two of them could be shown to satisfy the Poisson law. The others satisfied the binomial law or neither. 6. Within a given motoneurone the amplitude of an e.p.s.p. is closely related to the mean number of quanta released but not to the amplitude of the unit e.p.s.p. produced by the action of a single quantum of transmitter.
