In the primary visual cortex,activity of neurons evoked by stimuli within the classical receptive field (CRF) can be modulated by stimuli in the extra-receptive field (ERF).This modulating effect can be facilitatory (F-ERF) or inhibitory (I-ERF) that play different roles in visual information processing.Little is known on the cellular and neurotransmitter receptor bases of the two types of long-range interactions.Using in vivo intracellular injection technique,combined with the technique of immunocytochemistry,we have studied the morphological and immunocytochemical features of the F-ERF and I-ERF neurons.Significant differences were found in both the dendrite and axon structures,as well as the density distribution of excitatory and inhibitory neurotransmitter receptors,between the two types of neurons.