In layer 4 of cat visual cortex, the monocular, concentric receptive fields of thalamic neurons, which relay retinal input to the cortex, are transformed into 'simple' cortical receptive fields that are binocular and selective for the precise orientation, direction of motion, and size of the visual stimulus. These properties are thought to arise from the pattern of connections from thalamic neurons, although anatomical studies show that most excitatory inputs to layer 4 simple cells are from recurrently connected circuits of cortical neurons. We examined single fibre inputs to spiny stellate neurons. We examined single fibre inputs to spiny stellate neurons in slices of cat visual cortex, and conclude that thalamocortical synapses are powerful and the responses they evoke are unusually invariant for central synapses. However, the responses to intracortical inputs, although less invariant, are strong enough to provide most of the excitation to simple cells in vivo. Our results suggest that the recurrent excitatory circuits of cortex may amplify the initial feedforward thalamic signal, subserving dynamic modifications of the functional properties of cortical neurons.