CORRELATIONS between the firings of simultaneously recorded single units in auditory cortex may result from neural connectivity as well as covariation in firing rate. We present a predictor for the rate covariance part of the correlogram based on spike counts in 50 ms. Data were collected from 604 neurone pairs recorded under spontaneous conditions in primary auditory cortex of seven juvenile (30 70 days) and nine adult cats. A total of 341 pairs (56%) had a peak correlation coefficient significantly different from zero. The rate covariance comprised on average 73% of the peak correlation for single electrode pairs and 67% of that for dual electrode pairs. The remaining event correlation was significantly different from zero in 150 of the pair correlograms tested.