Studies were designed to evaluate bull sperm motility and membrane integrity following exposure to Tyrode's solution varying in osmolality from 100 to 1537 mOsm. Congo red and bisbenzimide (HOECHST 33258) stains were used to distinguish between sperm with intact versus disrupted plasma membranes. Sperm motility was subjectively evaluated. No significant differences were found between motile and unstained sperm (sperm with intact membranes) in solutions with nearly physiologic osmolalities between 200 and 300 mOsm. However, in 100- and 150-mOsm solutions, the percentage of motile sperm (5 and 19%), respectively) was lower than the percentage of sperm unstained with Congo red (18 and 35%). With HOECHST 33258 stain, the corresponding values were 7 and 14% versus 26 and 31%. The percentage of motile sperm declined greatly in the 500-mOsm medium, but the proportion of unstained sperm was affected little until the osmotic pressure exceeded 732 mOsm. Partial recovery of motility occurred when sperm were returned to the isotonic medium. This study indicated that the sperm plasma membrane was more resistant to osmotic damage than were the mechanisms responsible for sperm motility. Lack of motility in hypertonic media was not an absolute indicator of cell death, and unstained sperm overestimated sperm viability, which are factors to consider when sperm are being evaluated, especially relative to cryopreservation.