OBJECTIVE Our purpose was to measure central nervous system insult from prenatal phenobarbital by quantified electroencephalographic evaluation of mature rat offspring. METHODS Twenty-four adult female rats were given phenobarbital as 0, 20, 40, or 60 mg/kg/day subcutaneously between 9 and 10 AM for 28 days before breeding and throughout gestation. The electroencephalograms of 90-day-old offspring were sampled over 24 hours and converted to the power spectra. RESULTS Female offspring exposed prenatally to 20 mg and males exposed prenatally to 40 mg displayed an increase in delta and a decrease in alpha and middle-beta waveform activities. Females exposed prenatally to 40 mg and males exposed prenatally to 60 mg displayed decreases in delta, theta, alpha, and middle-beta activities. CONCLUSIONS Prenatal phenobarbital exposure produced dose- and sex-dependent changes in the electroencephalogram of mature offspring consistent with a generalized deficit in the neuronal synchronization mechanisms that are critical for normal sleep and learning.