The development of the electroencephalogram (EEG) and, especially, behavioral state-specific EEG patterns was studied in white and black hooded rats of the Lister strain, aged 9-30 days. Movements of the rat were recorded and the behavioral state was monitored by means of a push-button device. The EEG was collected when the rat was in State 1 (regular respiration, absence of movements, and, after the 14th day when the rat eye opens, eyes closed), State 2 (irregular respiration, continual occurrence of twitches, and, after the 14th day, eyes closed), and in State 4 (irregular respiration, presence of gross body movements, and, after the 14th day, eyes opened). The EEG of rats on the 9th and 10th day did not reveal behavioral state-specific patterns. The amplitudes of the EEG were low and only low frequencies occurred. Between the 10th and 13th day an EEG pattern specific for State 1 containing high amplitudes developed. From the 14th day onwards spindles (frequencies from 14-18 Hz) occurred in the EEG during State 1. From the 14th day onwards, the EEG from the visual cortex during State 2 showed a regular and continually occurring 5-Hz rhythm. Bursts with 5-Hz waves were recorded from the visual cortex only intermittently during State 4. The EEG frequencies during the bursts varied between 5 and 7 Hz from the 17th day onwards. Computer analysis of the amplitude distributions showed a considerable increase in the power after the 10th day. The analysis of the frequency spectra indicated that the power increase occurs expecially in the higher frequencies of the EEG signal. Visual analysis as well as computer analysis of the EEG did not reveal systematic changes in the EEG after the 18th day when the EEG was similar to that recorded at older ages.