1. We studied the effect of electrically stimulating the locus coeruleus (LC) and iontophoresing noradrenergic antagonists on visual responses and spontaneous activity of individual cells in the cat primary visual cortex. 2. A bilateral projection from LC to visual cortex was demonstrated anatomically, by retrograde labeling using horseradish peroxidase. Where electrical stimulation of both ipsilateral and contralateral LC affected a cortical neuron, the effect induced by stimulating each side was similar. 3. One hundred and two cells were recorded in area 17: 52% of them had their activity suppressed and 36% had their activity facilitated by LC stimulation. The suppressive effect was predominant in cortical layers II + III and IV, whereas most cells in layer V and one-half of the cells in layer VI were facilitated by LC stimulation. This suggests that LC neurons innervate each cortical layer in a different manner. 4. Simple and complex cells were equally sensitive to LC stimulation. For simple cells, the suppressive effect of LC stimulation was dominant throughout all layers. For complex cells, the suppressive effect was dominant in layers II + III and IV, whereas the facilitatory effect was dominant in layers V and VI. 5. The suppressive effect of LC stimulation was blocked by iontophoretic application of beta-adrenergic receptor antagonists and the facilitatory effect was blocked by either alpha- or beta-adrenergic receptor antagonists. 6. Nonselective alpha-, and selective alpha 1- and alpha 2-receptor antagonists suppressed visual and spontaneous activity in almost all neurons tested, suggesting that these receptors are either facilitatory at a postsynaptic site or inhibitory at a site presynaptic to an inhibitory synapse in the visual cortex. 7. beta-Receptor antagonists facilitated activity in 45% and suppressed activity in 36% of the cells tested, suggesting there are both suppressive and facilitatory types of beta-receptors. 8. The effectiveness of alpha- and beta-antagonists on the activity of neurons without LC stimulation also suggested that spontaneously released noradrenaline activated noradrenergic receptors in the visual cortex even in the anesthetized and paralyzed cat. 9. In most cells tested, both alpha- and beta-receptor antagonists exerted effects on single neurons suggesting that endogenous noradrenaline acts on both alpha- and beta-receptors on the same cell. 10. The activation of LC did not improve the signal- (visual response)to-noise (spontaneous discharge) ratio of neurons in the visual cortex. 11. LC seemed to control the activity of each cortical layer differently, by activating different kinds of noradrenergic receptors in different layers.