Ischaemic injury of developing white matter axons has great clinical relevance. For example, loss of axon function underlies the functional deficit in cerebral palsy, a common developmental disorder that is thought to be of ischaemic origin in many cases. We investigated the mechanism of acute injury of axons in a white matter tract that is just initiating myelination (perfused, isolated, optic nerve of humanely killed 8-12 day post-natal rat). In the widely studied adult rat optic nerve, ischaemia gates a toxic Ca²⁺ influx mediated by L-type voltage gated Ca²⁺ channels and reverse operation of the Na-Ca exchanger (Stys et al., 1992; Fern et al., 1995). The amplitude of the extracellular compound action potential (CAP) was used as a measure of the functional integrity of axons (see Fern et al., 1998). A standard protocol involved 10 min of control recording in ACSF followed by an 80 minute ischaemic insult (O₂ and glucose withdrawal), followed by a 60 min recovery period in ACSF. CAP amplitude collapsed to 4.5 ±1.7% (mean ±SEM, n=9) during the ischaemic period and recovered to 32.3 ±6.0% of the pre-ischaemic value during the recovery (P<0.001 by ANOVA). In time-matched controls (130 min ACSF perfusion), CAP amplitude remained relatively stable (94.5 ±3.7%, n=8). Removing Ca²⁺ from the perfusate (+50 μM EGTA) significantly improved recovery from ischaemia (64.4 ±5.6%, n=8; P<0.001), but did not achieve the complete protection seen in the adult. Removing Na⁺ from the perfusate to block operation of the Na-Ca exchanger reduced recovery to 7.1 ±10.4% (n=9; P<0.001). This suggests that the Na-Ca exchange protein does not contribute to ischaemic Ca²⁺ influx in neonatal axons, indeed it is more likely to be removing Ca²⁺. Block of L-type voltage gated calcium channels with diltiazem (50 μM) did not increase CAP recovery (45.1 ±7.8%, n=7). Neither Na⁺ removal nor perfusion with diltiazem had any significant non-reversible effect upon CAP area in time-matched controls. These results demonstrate distinct differences between the mechanisms of ischaemic injury in mature and immature rat optic nerve axons.