The EEG during the sleep of a 'new variant' (having a dihyropteridine reductase deficiency) phenylketonuric child, aged 11 months, has been studied under low phenylalanine and following a substitute treatment with 5-HTP and L-DOPA. Prior to treatment, a paroxystic type of activity composed of spikes and/or polyspikes associated with sharp theta waves was chiefly observed during paradoxical sleep (PS). After a 10 day treatment with 5-HTP and L-DOPA, the EEG was not modified. However, after 6 months of this therapy, the paroxysms disappeared as well as the spindles; the quiet sleep trace was disorganized. The EEG modifications observed could be due to an evolution of the illness, if the substitute treatment was begun too late to go into effect.