Six behaving cats were administered N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) (i.p. 10 mg/kg), a neurotoxic agent known to destroy noradrenergic endings in the CNS. They were placed, both before (control) and after treatment in 3 different situations, each time for 90 min: (1) a 'neutral' one (N) with no significant stimuli; (2) another one eliciting focused attention (F); (3) a third one, creating a situation of 'expectancy of an event to occur' (E), with the animal usually displaying a posture of quiet waking. Simultaneously, the electrocorticogram (ECoG) was recorded from the sensorimotor and parietal cortex through implanted electrodes. We already knew and were able to confirm that the normal animals (i.e. before treatment) displayed distinct behavioural patterns depending on the situation and distinct accompanying parietofrontal ECoG activities, with a dominance of drowsiness and sleep during N, that of a 36-Hz 'beta' rhythms in condition F, and of 14-Hz 'mu' rhythms in condition E. It was shown that the prevailing attitude of the animals after treatment was now, in all 3 situations, that of 'quiet waking and/or expectancy-like watching', with a large if not exclusive dominance of only one ECoG pattern, namely mu. These changes were considered as due to a release of the mu system from a noradrenergic modulatory blockade, in accordance with some of our previous data. An immunohistochemical study with anti-tyrosine hydroxylase antibody was also performed; it confirmed that after DSP4 treatment there were substantial alterations in the immunoreactivity of locus coeruleus cells, the structure which is likely to be involved in this NAergic control of the mu rhythms and of its concomitant behaviour.