Lipid peroxidation (LPO) of stallion spermatozoa was assessed in fresh semen and in samples of the same ejaculates after freezing and thawing. Particular attention was paid to individual differences in the susceptibility to LPO and its possible relationship with freezability. Innate levels of LPO were very low in fresh spermatozoa but increased after thawing, a change that was largely stallion-dependent. The level of LPO in fresh spermatozoa was not correlated with that of the thawed spermatozoa. Negative correlations existed between LPO and intact membranes post-thaw (r= -0.789, Pless than0.001), and also between LPO and spermatozoa with high mitochondrial membrane potential (Delta psi m) post-thaw (r= -0.689, Pless than0.001). LPO was also highly and significantly correlated with caspase activity. The correlation between caspase activity in ethidium positive cells and LPO was r=0.772, Pless than0.001. This LPO is unlikely to represent, per se, a sign of cryopreservation-induced injury, but it is apparently capable of triggering apoptotic-like changes that could result in the sub-lethal cryodamage often seen among surviving spermatozoa. Reproduction (2009) 138 55-63