Sixteen rats were chronically implanted with bipolar electrodes in the hippocampal regions containing cells generating electric theta-activity. The animals were divided into 4 groups of which 2 were exposed to 500 p.p.m. of toluene in inhalation chambers, for 8 or 16 hours per day for 5 days per week in 12 weeks respectively, and 2 served as controls. The hippocampal electric activity was recorded 48 hours after each weekly exposure, ensuring a minimal amount of toluene in the tissue during the recordings. EEG-recordings were read blindly by two experienced scientists, and the frequency of theta-waves in the exposed groups were compared to their respective control group at each recording by Student's t-test. Frequencies of theta-activity in the exposed groups were found to differ from their respective control group by variance analysis. Each point on the frequency versus time plot were further analysed by Student's t-test. Compared to the non-exposed group the eight hours daily exposed group showed an initial period of increased frequency of the regular theta-waves together with an increased incidence of theta-activity after 1-2 weeks of exposure. In the sixteen hours daily exposed rats two weeks of toluene inhalation produced a significant reduction in the theta-wave frequency. This change was also reached after eight weeks of exposure in the eight hours daily exposed group. At this moment the theta-activity was frequently disrupted by short amplitude irregular waves, a phenomen which increased gradually throughout the rest of the exposure period. The average blood concentration of toluene was 16.7 micrograms/ml and 17.7 micrograms/ml and not significantly different for the eight and sixteen hours exposed groups respectively.