1. Spontaneous activity of single afferent neurones in the cochlea of the guinea‐pig was studied by measurement of mean rates and computation of interspike interval histograms. 2. The shapes of interval histograms agreed with those found in cat cochlear nerve fibres. Modes of the histograms were less than 7 msec for all cells with mean rates ranging from 20 to 150 spikes/sec. 3. The distribution of mean rates showed higher maximum rates than those found in cat. Cells from animals with abnormally high thresholds showed lower mean rates than those from sensitive animals. 4. No correlation was found between mean spontaneous rate and sensitivity to acoustic stimulation. No discrete populations of cells could be discerned on any of the criteria used. 5. The simultaneous effects of hypoxia on cell thresholds, mean spontaneous rate and scala media potentials are reported. These and other results are discussed in relation to possible sites of generation of spontaneous activity in primary auditory afferents.