All rats of the WAG inbred strain show electrophysiological and behavioral phenomena reminiscent of human absence epilepsy. To study the genetic architecture of this kind of epilepsy, WAG rats were cross bred with inbred ACI rats which show no signs of epilepsy. Number and duration of spike-wave discharges per hour were determined from 24-h recordings of cortical EEG in parental strains and reciprocal F1 hybrids. All hybrids showed spike-wave discharges, indicating complete dominance for occurrence, but different genetic backgrounds were suggested for number and duration of the phenomena. These results imply that more than one gene is involved in absence epilepsy. Some genes determine the occurrence, while others may manipulate the actual number and duration of the epileptic phenomena.