The effect of centrifugation of diluted and undiluted semen on equine and bovine spermatozoan motility and fertility was examined, as was the effect of seminal plasma and dilution on stallion spermatozoa during incubation before and after freezing. Centrifugation at 370 g or 829 g was not detrimental (P greater than 0.05) to prefreeze or postfreeze motility if a final concentration of 10% seminal plasma was present. A reduction of seminal plasma from 10% to 2% significantly (P smaller than 0.05) reduced motility. A centrifugal force of 956 g significantly reduced prefreeze but not postfreeze motility of spermatozoa in undiluted semen, regardless of seminal plasma concentration. With a dried skim milk extender, prefreeze and postfreeze motility was greater in samples containing 20% seminal plasma. Motility was depressed by high and low concentrations of seminal plasma. The fertility of frozen or unfrozen stallion spermatozoa was not depressed (P greater than 0.05) by centrifugation at 310 g for 3.5 minutes. In contrast, the fertility of bull semen was significantly (P smaller than 0.05) lowered by centrifugation at 270 g for three minutes. Further, the fertility of centrifuged, diluted bovine semen was lower (P smaller than 0.05) than centrifuged, undiluted semen.