The effect were examined in vitro of the ionic composition of the incubation medium on the acquisition and loss of the potent ial for motility were exammined by the spermatozoa of the Japanese eel ( Anguilla japonica) . Milt was obtained from male eels that had been artif icially matured by repeated injections of HCG. The percent motility increased with increasing concentrations of Na+ 、K 、Ca2 and Mg2 ions ( 0- 400mmol#L- 1 ) and decreased with concentrations of those ions( 500- 1000 mmol#L- 1) , eel sperm was collected and then incubated w ith nitric oxide donor-sodium nitroprusside in different doses. Eel sperm motility was accelerated or maintained at low dose of sodium nitroprusside ( SNP) ( 10- 10- 10- 6mol#L- 1) , while marked inhibition effects were found at high dose ( SNP> 10- 5mol#L- 1) . Nitric oxide affect s the motility of eel sperm in dose- dependent manner. The eel sperm of maturation by SPN and HCG was used for fertilizing three batches of eggs and hatchability is higher than the sperm of maturation by HCG.