ExtractSleep behavior and bioelectric brain development have been studied by means of polygraphic recordings (respiration, eye movements, muscle activity, and electroencephalogram (EEG)) in 22 infants and young children with phenylketonuria (PKU). The EEGs were analyzed by a special visual pattern recognition program as well as by means of computer spectral analysis. The distribution between rapid eye movement (REM) or active and non-REM (NREM) or quiet sleep was not found to be different from control infants of the same age. Quite contrary to current theories on the biochemical regulation of sleep as derived from acute animal experiments, our results indicate that, under chronic conditions, a normal sequence of quiet and active sleep can be maintained despite a severe lack of blood and cerebrospinal fluid serotonin as it occurs in hyperphenylalaninemia. In the EEG of subjects with PKU the development of a rhythm or sleep spindle (12-16 cps) activity was enhanced and already abnormal as early as 5 weeks after term birth. The frequency of a waves as well as of hypnagogic, monomorphic, 6 activity was shifted toward higher values. Dietary treatment of 4-6 weeks duration and normalization of blood phenylalanine levels did not significantly change the abnormal bioelectric phenomena. SpeculationThe increase in high frequency regular rhythmic activity in the sleep EEG of subjects with PKU can be explained by decreased concentrations of inhibitory synaptic transmitters such as serotonin and 7-aminobutyric acid. The developmental course in the expressivity of sleep spindles during the 1 st year of life, together with reliable methods of quantified EEG analysis, seems to be a rather sensitive indicator of normal or abnormal brain maturation. Introductionshould be, in theory, alterations of sleep behavior be-