Diluents for cool storage of milt and for artificial fertilization which induce a high percentage of motility of stored spermatozoa were examined. Milt was stored in artificial seminal plasma （ASP; NaCl+KCl=140 mM, CaCl2 1.6 mM, MgCl2 1.0 mM and NaHCO3 10 mM buffered with 20 mM TAPS-NaOH at pH 8.0） of various KCl concentrations （0 to 50 mM） at 4℃ for 10 days. The sperm motility was examined after dilution with DW （buffered with 10 mM TAPS at pH 8.0） or 120 mM NaHCO3. Increasing the potassium concentration of ASP to 30 mM increased the percent motility. The spermatozoa stored for two days in Ca-free ASP showed remarkably decreased motility. The motility of stored spermatozoa is also influenced by the pH of ASP. A 120 mM NaHCO3 solution induced a higher percentage and longer duration of motility of stored spermatozoa than DW. A 120 mM NaCl solution and an artificial coelomic fluid also induced a higher percent motility; no significant differences in motility were detected among these three solutions. Adding 5 mM IBMX （3-isobutyl-1-methylxanthine） to 120 mM NaHCO3 or 120 mM NaCl failed to increase the percent motility. Changing NaHCO3 concentration from 60 to 180 mM induced no significant difference of percent motility either 5 to 7 sec or 15 to 17 sec after the dilution.