Coronal slices from guinea pig visual neocortex were stained with voltage-sensitive fluorescent dyes RH414 or RH795. Activity was evoked by electrical stimulation of either white matter or layer I. Emitted-light intensity changes representing summated changes of membrane potential were recorded by a 10 x 10 photodiode array with a temporal resolution of 0.4 ms and a spatial resolution of 60 microns or 94 microns. Following either stimulation of layer I or of white matter, maximal activity was located close to the respective stimulation electrode, in upper layer III/II, and between layer IV and V. With stimulation of the white matter, additional peak activity was recorded from upper layer VI. Non-synaptic activity was separated from mixed (synaptic and non-synaptic) activity by comparing responses obtained in standard perfusion medium with those obtained in perfusion medium from which the calcium was omitted, such that synaptic transmission was blocked. With stimulation of the white matter, most of the evoked activity in lower cortical layers was of non-synaptic origin. This non-synaptic activity consisted of early and fast potentials, which were predominant in layer VI and probably represented presynaptic fibre activity, and of slower components that were presumably of antidromic origin. Significant postsynaptic activity was only found in upper layer III/II. In contrast, with stimulation of layer I, most of the evoked activity was of postsynaptic origin. Early and fast non-synaptic potentials consisting of presynaptic fibre activity were confined to layer I. Slower non-synaptic activity, that might reflect direct dendritic activation, was minimal and was confined to upper cortical layers. Thus, following either stimulation of layer I or of white matter, the major postsynaptic components were found in upper layer III/II. It is suggested that the postsynaptic response following stimulation of white matter resulted from di- or polysynaptic activation by afferent fibres. The postsynaptic response to stimulation of layer I was presumably a monosynaptic activation of apical dendrites from pyramidal cells by layer I horizontal fibres. Activity following stimulation of white matter spread faster than activity following stimulation of layer I. This might reflect the difference in active conduction along afferent and efferent fibres on the one hand and in passive conductance along the dendritic tree on the other hand.