Changes in intrinsic optical signal of rat neocortical slices following afferent stimulation were recorded using darkfield infrared-videomicroscopy. Response amplitude was linearly related to stimulation intensity. The intensity of the optical signal reached its maximum 3 s after onset of stimulation and redecayed with a mean time constant of 23 +/- 7.1 s. The optical signal had a columnar shape. The size of the column was independent from stimulation intensity with stimuli of medium amplitudes. The extent of the optical signal corresponded to the extent of the electrical activation. Changes in intrinsic optical properties may be a useful tool for the study of spread of excitation in neuronal tissue in vitro.