In summary, despite semen being stored for 4 days in extender containing 50% SP yielding low values for sperm motility, SMI and fertility were not adversely affected for this stallion. These findings stress the importance of assessing sperm plasma membrane integrity for evaluating semen quality in addition to other semen characteristics, and SMI may prove to be one of the more important criteria for predicting potential fertility of cool-stored semen in stallions.