We have investigated the responses of neurones in the guinea-pig superior colliculus to combinations of visual and auditory stimuli. When these stimuli were presented separately, some of these neurones responded only to one modality, others to both and a few neurones reliably to neither. To bimodal stimulation, many of these neurones exhibited some form of cross-modality interaction, the degree and nature of which depended on the relative timing and location of the two stimuli. Facilitatory and inhibitory interactions were observed and, occasionally, both effects were found in the same neurone at different inter-stimulus intervals. Neurones whose responses to visual stimuli were enhanced by an auditory stimulus were found in the superficial layers. Although visual-enhanced and visual-depressed auditory neurones were found throughout the deep layers, the majority of them were recorded in the stratum griseum profundum. Neurones that responded to both visual and auditory stimuli presented separately and gave enhanced or depressed responses to bimodal stimulation were found throughout the deep layers, but were concentrated in the stratum griseum intermediale and extended into the stratum opticum.