Abstract
1. Recordings were made from sixty-five cells in the visual cortex of unanaesthetized, dark-adapted cats and transferred automatically to computer input paper tape.2. The activity of each cell was measured as a function of time (the running mean). The unit of time used was inversely proportional to the mean firing rate, in order to give comparable results for different cells.3. For sixty-nine sections of discharge from fifty-two cells, the length of time recorded was sufficient to test for the long-term stability of the running mean. In twenty-six sections, various kinds of trend and long-term irregularity were found.4. The interspike interval histogram was computed for the forty-one sections (from thirty-one cells) in which there were more than 2000 discharges. Only eight histograms approximated closely to the exponential form. A test was also made of the distribution of the longer intervals alone and in twenty-five sections they did not deviate significantly from the exponential form.5. There was no significant correlation between the behaviour of the longer interspike intervals and the long-term stability or otherwise of the running mean.