The present study concerns the interactions of local pre/postsynaptic covariance and activity of the cortically-projecting cholinergic basal forebrain, in physiological plasticity of auditory cortex. Specifically, a tone that activated presynaptic inputs to a recorded auditory cortical neuron was repeatedly paired with a combination of two stimuli: (1) local juxtacellular current that excited the recorded cell and (2) basal forebrain stimulation which desynchronized the cortical EEG. In addition, the recorded neurons were filled with biocytin for morphological examination. The hypothesis tested was that the combined treatment would cause increased potentiation of responses to the paired tone, relative to similar conditioning treatments involving either postsynaptic excitation alone or basal forebrain stimulation alone. In contrast, there was no net increase in plasticity and indeed the combined treatment appears to have decreased plasticity below that previously found for either treatment alone. Several alternate interpretations of these results are discussed. © 2001 Elsevier Science B.V.