We have previously demonstrated that activation of group III metabotropic glutamate receptors (mGluRs) facilitates spontaneous glutamate release in rat entorhinal cortex (EC) in vitro (Evans et al. 2000), and group II mGluRs have a similar effect (Ayman et al. 2003). This uncharacteristic effect of group II and group III mGluRs was recorded in slices prepared from young animals (4-6 weeks). Since developmental changes in expression and function of these receptors have been demonstrated (e.g. Ross et al. 2000), it was of interest to determine if there were changes in the effects of the mGluRs in the rat EC with age. Slices of EC were prepared from humanely killed male Wistar rats aged 4-6 weeks or 5-6 months. Glutamate release was monitored by recording spontaneous excitatory postsynaptic currents (sEPSCs) from neurones in layer V. All recordings were made in the presence of the NMDA receptor blocker, MK-801 (10 μM), so EPSCs reflect activation of AMPA receptors. Statistical comparisons were made using either the Wilcoxon rank sum test (WRST) of the Kolmogorov-Smirnoff test (KS). Mean interevent interval (IEI) for sEPSCs recorded from 16 cells in the young group was 312.5±7.1 ms. Whereas, in the older animals mean IEI (n = 14) it was 21 % less at 246.5±6.6 ms (P < 0.05, WRST). The mean amplitude of events was 13.4±0.2 pA for the older group and 11.1±0.1 pA for the young group (P < 0.05, WRST). In the case of miniature (m)EPSCs, recorded in TTX, mean IEI was over 200% greater in the young group (896.4±32.9 ms, n = 7) than in the older animals (287.6±10.8 ms, n = 4; P < 0.05, WRST). Thus, in the older animals, EPSCs were larger, more frequent and have a smaller contribution from action potential-driven release. The effects of the group III mGluR agonist, ACPT-1, were tested on 8 layer V neurones recorded in slices from older animals. Mean IEI under control conditions was 265.7±9.5 ms and mean amplitude was 12.6±0.2 pA. In the presence of ACPT-1 (20 μM), mean IEI increased to 357.3±10.3 ms (P < 0.001, KS). Mean amplitude was unchanged. Subsequent addition of DCG-IV (5 μM), a group II mGluR agonist, increased IEI further to 520.8±15.1 ms (P < 0.001, KS), without change in amplitude. Thus, in addition to the overt changes in spontaneous glutamate release seen in the older animals, there was a reversal of the effects of group II and group III mGluRs, both now eliciting a depression of release in contrast to the facilitation seen in younger animals. Whether this reflects developmental differences in the receptors per se, or whether it is due to intrinsic changes in the release mechanism is a subject for further investigation.