Activity of neuronal pairs in the inferior colliculus of the rabbit was recorded with a single stainless-steel microelectrode. Seventy pairs were investigated with monaural and binaural tonal stimuli. The most common parameter of the response in neuronal pairs was the best frequency, which was similar in 100% of the pairs (n = 45). Q10 values were identical in 44% of pairs and threshold tuning curves in 27% of pairs. Units with a smaller spike amplitude usually had a shorter latency to both binaural and monaural stimuli, when measured 10-20 dB above the best frequency threshold. Most units discharged during the entire period of the 100 ms tone stimulation at their best frequency; large differences, however, were found in their firing pattern, when peristimulus histograms were compared. High correlation was found in pairs where both neurones exhibited the same type of binaural interaction. The following types of binaural interaction were found: binaural excitatory drive with occlusion, binaural excitatory drive with facilitation; monaural excitatory drive with inhibition from the other ear and pure monaural excitatory drive. In a significant number of neuronal pairs the influence of binaural stimulation was similar for both neurones. The results suggest that: (a) many adjacent neurones in the inferior colliculus convey parallel information concerning features of the auditory stimulus; (b) units with a similar type of binaural interaction may be organized in clusters within isofrequency layers.