Because recalcitrant seeds are not desiccation-tolerant they must be stored moist. Their limited storage potential presents significant practical problems, but the cause of viability loss is not known. It has been suggested that a stress-induced metabolic imbalance can develop during storage that results in free-radical generation and consequent damage. To investigate this hypothesis, the presence of a stable free radical, lipid peroxidation and representative enzymatic and nonenzymatic protection mechanisms against oxidative attack were monitored in nondormant recalcitrant seeds during moist storage. A comparison was made between seeds of a short-lived sub-tropical species (Avicennia marina) and two longer-lived temperate species (Quercus robur and Castanea sativa). As a test of the hypothesis, seeds of both temperate species were held under conditions of elevated temperature and oxygen concentration to develop different rates of respiration during storage. The number of normal seedlings produced from seeds of the two temperate species declined during storage, but viability remained high, so effects of ageing were not confounded with an increasing proportion of dead seeds in the population. Under these conditions, lipid peroxidation changed little over the storage period, although there was evidence of accumulation of a stable free radical in Q. robur axes. However, this response was not affected by storage conditions that elevated respiration rates. In the shorter-lived A. marina seeds viability declined soon after the start of storage, but the significant increase in free radicals shown by EPR measurement was only evident when an increasing percentage of the seed population was no longer viable. Changes in the activity of scavenging enzymes and the concentration of antioxidants were time-dependent and not related to respiration rates. Therefore, in the present work, no consistent evidence was found to show that metabolism-induced free-radical activity was a significant contributing factor to pre-mortem deterioration in moist-stored recalcitrant seeds.