FIELD potential recordings were made from the dentate gyrus of urethane-anaesthetized rats in order to investigate the ability of 5 Hz stimulation to reverse long-term potentiation (LTP) induced by a high frequency tetanus. A 10 min train of 5 Hz was found to reverse LTP in a time-dependent fashion: as the interval between tetanus and 5 Hz was increased, LTP became progressively less susceptible to reversal. If 10 min or 30 min intervened between tetanization and 5 Hz stimulation, LTP was unaffected. These results indicate that dentate LTPin vivoexhibits a similar limited time window of vulnerability to reversal by low frequency stimulation to that previously reported in area CA 1in vitro.