Although modulation of hippocampal synaptic plasticity by neurotrophins or growth factors has recently become an extensively investigated subject, there are no reports arguing for the contribution of the transforming growth factor (TGF)-beta superfamily. In the present study, we examined the effect of activin, a member of the TGF-beta superfamily, on long-term potentiation (LTP) of the dentate gyrus in anesthetized rats. Activin significantly impaired the formation of LTP induced by tetanic stimulation (60 Hz for 0.27 sec), but not that by strong tetanic stimulation (60 Hz for 0.5 sec). These results suggest that activin selectively blocks the induction of LTP evoked by threshold tetanic stimulation.