The responses of visual cortical neurons during iontophoresis of norepinephrine (NE) were compared in kittens and adult cats. One type of neuronal "plasticity" (the ability of visual cortical neurons to have their response properties markedly altered during exposure to a restricted visual environment) normally occurs only in young kittens and has been hypothesized to depend on the presence of NE. We investigated this hypothesis in terms of a model of synaptic plasticity which suggested that NE would selectively enhance the evoked responses of neurons and that this enhancement might exist to a greater extent in kittens than in adult cats. The responses of 72% (43 of 60) of all neurons sampled decreased during NE iontophoresis, while the responses of only a few neurons (3 of 60) increased. Reduced activity occurred in roughly equal percentages of simple and complex cells. We also looked for differences in NE modulation of the "signal-to-noise" ratio of neurons. Most neurons had little spontaneous activity, but, of those that did, the signal-to-noise ratio increased in some instances and decreased in others. No difference in any of the effects of NE was observed between kittens and adult cats. The role of NE in cortical plasticity is discussed.