To investigate the detailed actions of transforming growth factor-beta (TGF-beta) in the brain, which increase accompanied with continuity of exercise, the authors performed electroencephalogram (EEG) spectral analysis for 2 h after intracranial injection of TGF-beta in rats and compared with the effects of swimming exercise. Relative power values (power percent) of the theta frequency band (4-7 Hz) increased and power percent of the alpha frequency band (7-13 Hz) decreased after intracranial injection of TGF-beta. The directions of these changes of EEG after intracranial injection of TGF-beta were consistent with those after exercise. The EEG pattern produced by leucine-enkephalin (Leu-enk), a typical brain peptide related to exercise, was completely different from that after exercise. The results suggested that the increase in TGF-beta concentration in the brain is, at least partly, relevant to the change of neuronal activity after exercise.