Disruption of both alleles of the prion protein gene, Prnp, has been shown repeatedly to abolish the susceptibility of mice to developing prion diseases. However, conflicting results have been obtained from phenotypic analyses of prion protein (PrP)-deficient (Prnp0/0) mice lines. To explore the possible neurophysiological properties associated with expression or absence of the normal isoform of the cellular prion protein (PrPC), we used conventional in vitro extracellular field potential recordings in the hippocampal CA1 area of mice from two independently-derived Prnp0/0 strains. Basal synaptic transmission and a short-term form of synaptic plasticity were analysed in this study. Results were compared with animals carrying a wild-type mouse PrP transgene to investigate whether PrP expression levels influence glutamatergic synaptic transmission in the hippocampus. There was a clear correlation between excitatory synaptic transmission and PrP expression; i.e. the range of synaptic responses increased with the level of PrPC expression. On the other hand, the probability of transmitter release, as assessed by paired-pulse facilitation, appeared unchanged. Interestingly, whereas the overall range for synaptic responses was still greater in older mice over-expressing PrPC, this effect in these animals appeared to be due to better recruitment of fibres rather than facilitation of synaptic transmission per se. Taken together, these data are strong evidence for a functional role for PrPC in modulating synaptic transmission.