The representation of voice onset time (VOT) for 197 single units in cat primary auditory cortex was studied for a /ba/-/pa/ continuum in which VOT was varied in 5-ms steps from 0 to 70 ms. The effect of stimulus intensity, characteristic frequency of the neurons, and age of the animals was investigated. The minimum VOT represented in onset responses to both the voiceless and voiced parts of the sound (a "double-on" response) was dependent on overall stimulus level. An interaction was found between the efficacy of the burst in evoking neural activity and the size of the subsequent response to the onset of voicing. There was only a minor difference in the mean values for the minimal neural VOT for young (42 ms), juvenile (36 ms), and adult animals (46 ms), albeit that for individual young and juvenile animals more frequently values close to 10-15 ms were found. The cumulative distribution for the adult group showed a relative lack of neural VOTs around 30-40 ms. No other cues in the single unit and local neuronal group firing rate representation of VOT were found that were related to the categorical perception boundary.