Activity-dependent synaptic plasticity was examined in vivo in two cortical areas of the adult monkey visual system, the primary visual and inferotemporal cortex, the first and late cortical stages essential for object recognition. Discontinuous high-frequency electrical stimulation of intrinsic horizontal connections in layer 2/3 caused contrasting forms of synaptic plasticity in the two areas. In the inferotemporal cortex, long-term potentiation of extracellular field potentials in layer 2/3 was induced, whereas in the same pathway of V1, identical stimulation protocol elicited long-term depression. The results indicate that susceptibility to synaptic plasticity varies among cortical areas in the monkey brain.