EEG activity in the 35--85 c/sec range has been reported from the rhinencephalon of numerous species of homeotherms, and from numerous parts of the forebrain in carnivores and primates, including man. Unimodal distributions of frequencies within this range for each of 3 species (cat, rat and rabbit) are reported here with proportionality between means and variances. Theoretical analysis has shown that the basis for this oscillatory activity lies in the feedback synaptic interactions of assemblies of excitatory and inhibitory neurons, and that there is a neural basis for frequency convergence and high amplitude near 40 c/sec. Given the synaptic mechanisms, the unimodal spectral distributions, and the widespread occurrence of EEG activity in this range, it clearly represents a unique and identifiable form of brain activity.