The effects of fenfluramine on electrophysiological activity in autistic children were assessed by computerized EEG power analysis and brainstem evoked responses (BSER). Prolonged administration of fenfluramine resulted in a decrease of relative share of α-waves in the EEG power spectrum and in corresponding increase in slow activity. No effects of the drug on BSER latencies were observed. The spectral EEG pattern of fenfluramine identified in this sample was compared to the spectra reported for normal volunteers on an acute dose of fenfluramine. The implications of these findings for EEG classification of fenfluramine and neurochemical mechanisms of autism are discussed