This study was conducted to: (a) determine the optimum extender system for two extenders that will maintain the highest level of cellular integrity when stored at either 5C or 15C for a minimum of 72 hr; (b) evaluate the fertilizing capacity of stored spermatozoa using the optimum extender system; (c) critically analyze enzymatic and morphological changes associated with storage and aging of boar spermatozoa; and (d) characterize properties of boar spermatozoa important to fertilization. The least-squares means for the percentage of normal apical ridge acrosomes were significantly affected by ejaculate within boar (P< .001), boar x extender (P< .05), boar x storage temperature (P< .01), extender x storage temperature (P<.01), extender x cooling rate (P< .05) and antibiotic x storage temperature (P< .01). The regression equations of storage time x treatment effect were reported. The least-squares means for acrosin activity were significantly affected by ejaculate within boar (P<.001), ejaculate within boar x antibiotic (P< .05), ejaculate within boar x extender (P<. 001), ejaculate within boar x storage temperature and extender x storage temperature (P< .05). The regression of storage time x storage temperature was reported. The least-squares means for progressive forward motility were significantly affected by extender (P< .05), antibiotic (P< .05), ejaculate within boar (P< .001), antibiotic x extender (P< .05) and extender x storage temperature (P< .01). The regressions of storage time x treatment effects were reported. The least-squares means for vibrational and/or rotational motility were affected by ejaculate within boar (P< .001), ejaculate within boar x extender (P<.001), ejaculate within boar x cooling rate (P<.05), antibiotic x extender and extender x storage temperature (P< .01). The optimum extender system consisted of Purdue extender containing penicillin/streptomycin and cooled to a 15C storage temperature in 4 hours. This extender system maintained a 70% minimum fertilization rate for 84 hours. / Ph. D.