NMDA receptor-dependent long-term depression (LTD) is a major form of synaptic plasticity in the brain that is involved in learning and memory. This form of synaptic plasticity involves an alteration in the efficiency of synaptic transmission mediated by AMPA receptors and depends on NMDA receptor activation. However, little is known concerning the signalling mechanisms leading from NMDA receptor activation to the alteration in AMPA receptor function. We have therefore investigated the potential role of protein kinases in LTD of glutamatergic synaptic transmission in the CA1 hippocampal region. Recently we found that, of approximately 60 ser/thr protein kinases investigated, only GSK-3β (glycogen synthase kinase-3 beta) is involved in LTD (Peineau et al, 2009). We have now investigated the role of tyrosine (tyr) protein kinases in LTD at the CA3:CA1 synapse of hippocampal slices obtained from two-week-old Wistar rats, killed in accordance with the UK Animals (Scientific Procedures) Act 1986. Using whole-cell patch-clamp recordings from CA1 pyramidal cells and tyr kinase inhibitors loaded into the recorded neurones, we have identified the involvement of the Janus kinase (JAK) family in LTD. Thus, NMDA-receptor dependent LTD (63 ± 2% of baseline; n = 28, see Peineau et al, 2009) is completely blocked by three different JAK inhibitors: CP690550 (1 µM, 101 ± 2 %; n = 5; p < 0.05 compared to control cells), AG490 (10 µM, 97 ± 3 %; n = 6; p < 0.05) and JAK inhibitor I (0.1 µM, 99 ± 2 %; n = 6; p < 0.05, one-way ANOVA). In contrast, two src-family tyr kinase inhibitors, PP2 (10-20 µM) and SU6656 (10 µM) had no effect on LTD (64 ± 3%; n = 7 and 64 ± 3%; n = 11, respectively). As JAK2 is the main JAK isoform expressed in the post-synaptic density, we focused on this isoform. We found, using western-blot analysis, that JAK2 is activated in hippocampal slices following NMDA treatment (20 µM, 3 min) and in CA1 dendritic mini-slices following low frequency stimulation (1 Hz, 15 min). Using tyr 1007/1008 phosphorylation of JAK2 as a measure of its activation status, we found an increase of 56 ± 10% (n = 6, p < 0.05) and 60 ± 9% (n = 4, p < 0.05; paired Student’s t test), respectively. Collectively, these data suggest that activation of JAK2 is required for NMDA receptor-dependent LTD in the hippocampus.