BACKGROUND: Nitric oxide (NO) has been shown to be important in sperm function, and the concentration of NO appears to determine these effects. Studies have demonstrated both positive and negative effects of NO on sperm function, but have not been able to provide a clear link between NO concentration and the extent of exposure to NO. To study the relationship between nitric oxide and sperm capacitationin vitro, and to provide a theoretical basis for the use of NO-related preparations in improving sperm motility for in vitro fertilization, we investigated the effects of NO concentration and time duration at these concentrations on in vitro sperm capacitation in both normal and abnormal sperm groups. We manipulated NO concentrations and the time duration of these concentrations using sodium nitroprusside (an NO donor) and NG-monomethyl-L-argenine (an NO synthase inhibitor). RESULTS: Compared to the normal sperm group, the abnormal sperm group had a longer basal time to reach the appropriate concentration of NO (p < 0.001), and the duration of time at this concentration was longer for the abnormal sperm group (p < 0.001). Both the basal time and the duration of time were significantly correlated with sperm viability and percentage of progressive sperm (p < 0.001). The experimental group had a significantly higher percentage of progressive sperm than the control group (p < 0.001). CONCLUSIONS: We hypothesize that there is a certain regularity to both NO concentration and its duration of time in regards to sperm capacitation, and that an adequate duration of time at the appropriate NO concentration is beneficial to sperm motility.