Excitatory neurotransmission at the N-methyl-D-aspartate (NMDA) receptor is selectively blocked by 2-amino-7-phosphonoheptanoate acid (2-APH). Acute focal microinjection of 2-APH into the rat hippocampus partially protects against cytopathology developing in selectively vulnerable neurons after 30 min of ischaemia and 2 h of reperfusion. We show that this protective action of 2-APH does not involve alterations in local cerebral blood flow (CBF). Intermediate cytopathology and long-term neuronal survival has been assessed in rats receiving focal injections of (+/-) 2-APH, 20 micrograms in 1 microliter, into one dorsal hippocampus prior to and 4 and 10 h after a 10-min period of forebrain ischaemia. Cytopathology assessed 4 or 24 h after ischaemia shows no difference between the buffer and 2-APH-injected hippocampi. Assessment after 7 days survival shows a significant protection against neuronal loss in the CA1 zone of the 2-APH-injected hippocampus compared with the contralateral, buffer-injected hippocampus. Systemic injection of D(-)2-APH (675 mg/kg i.v. at 0 h, 4 h, and 10 h) affords significant protection to CA1 hippocampal neurones (as assessed after 7 days). These results suggest that maintained blockade of neurotransmission at the NMDA receptor in the postischaemic period can protect against delayed cell loss. The mechanism may be through antagonism of the excitotoxic action of an endogenous neurotransmitter acting in the postischaemic period.