To investigate whether hippocampal synaptic transmission is enhanced during learning, we recorded synaptic field potentials in the dentate gyrus in response to stimulation of the perforant path in rats exploring a novel environment. Because these signals rapidly grow during brain temperature elevation, caused by any motor activity, the potentials were compared with signals sampled at similar brain temperatures after passive warming. Both the field excitatory postsynaptic potential (f-EPSP) and the population spike increased significantly early in the exploration, relative to temperature-matched control potentials. The effect decayed within 15-30 minutes. This is the first demonstration of a temperature-independent synaptic potentiation in the hippocampus during learning about the environment. The time course is similar to that of short-term potentiation.