The Ca²⁺-induced mitochondrial permeability transition pore (MPTP) plays a key role in the development of irreversible injury upon reperfusion of hearts following ischaemia. The MPTP can be inhibited in both isolated mitochondria and whole hearts by cyclosporin A (CsA), and this drug can protect adult hearts from reperfusion-induced damage. However, metabolism and mitochodrial function are very different in neonatal hearts, and the susceptibility of these heats to the MPTP has not been studied. Male Wistar rats were humanely killed, and mitochondria isolated from hearts of neonatal (2-3 day old), 7, 14, 21 day old, and adult (2-3 month old) animals. Pore-opening in response to Ca²⁺ was determined spectrophotometrically by monitoring the decrease in absorbance at 520 nm (‘swelling’). Mitochondria were incubated overnight for 12-18 h on ice prior to the assay (‘aged’) to ensure greater consistency because, during the first few hours after isolation, adenine nucleotides may change rapidly which will affect pore opening. ATP and ADP levels were measured in mitochondria from all ages, both in freshly isolated and aged mitochondria. This was achieved with the use of firefly luciferase and measured on a scintillation counter. n = at least 6 separate preparations of mitochondria. Data were analysed by 2-way ANOVA to determine statistical significances. Table 1 shows that the rates of pore-opening increased with decreasing age at both 10 μM Ca²⁺ (p<0.001) and 50 μM Ca²⁺ (p<0.001). In addition, whereas CsA almost abolished pore-opening in the adults at [Ca²⁺] up to 100 μM, in the neonate CsA was only inhibitory up to 10 μM Ca²⁺. Further investigation into the mitochondrial content of the ADP and ATP was carried out and it was found that both nucleotide levels were significantly lower in the neonatal mitochondria and increased with age. This confirms the increased pore opening of these mitochondria, even in the presence of CsA, as both ATP and ADP are known to inhibit MPTP opening (1). Thus, neonatal hearts may be much more susceptible to the damage caused by the MPTP upon reperfusion, which will need to be taken into account when designing cardioprotective strategies.