The neuroactive 4-10 fragment of adrenocorticotropin (ACTH 4-10) has been found to impair electroencephalographic (EEG) signs of selective attention in previous studies. It was hypothesized that this effect reflects a more general influence of the peptide weakening the mutual inhibition among frontocortical neuronal networks. Therefore, ACTH 4-10 was expected to loosen attentional control not only over external input but also over internal thoughts. This study examined the effects of ACTH 4-10 on the dimensional complexity of the EEG recorded while subjects solved tasks of convergent analytical thinking and of divergent creative thinking and during mental relaxation. Subjects were tested 30 min after i.v. administration of placebo or ACTH 4-10 (2 mg). ACTH 4-10 enhanced dimensional complexity of the EEG. The effect primarily concerned frontocortical recordings during convergent thinking, which, following placebo, was associated with the lowest EEG dimension. ACTH 4-10 also impaired behavioral performance on tasks of convergent thought, when presented verbally. Results suggest that ACTH 4-10 counteracts the inhibitory control among cortical neuronal networks necessary for orderly analytical thinking.