A form of long-term depression (LTD) of synaptic transmission can be induced by bath application of the group I metabotropic glutamate (mGlu) receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). The mechanisms responsible for the induction and expression of DHPG-induced LTD in the CA1 region of the hippocampus are currently the subject of intense investigation. Here we show that two protein tyrosine kinase (PTK) inhibitors (10 μM lavendustin A or 30 μM genistein) have little effect on DHPG-induced LTD. In contrast two protein tyrosine phosphatase (PTP) inhibitors (1 mM orthovanadate or 15 μM phenyl–arsine oxide) significantly inhibited DHPG-induced LTD. These data suggest that DHPG-induced LTD involves activation of a protein tyrosine phosphatase.