Spermatozoa of freshwater Cyprinidae (goldfish, carp, crucian carp and dace) remained immotile when the semen was diluted in solutions of NaCl, KCl, mannitol or glucose iso-osmolar to the seminal plasma (300 mosmol kg-1). The spermatozoa became motile in media containing these solutes if the osmolality was lower than that of the seminal plasma, suggesting that motility is suppressed by the osmolality of the seminal plasma in the sperm duct and initiated by a decrease of osmolality upon spawning into fresh water. Potassium was a major component of seminal plasma, having a concentration 20-30 times higher than that in the blood plasma in goldfish and carp. Sodium concentration in seminal plasma was lower than that in blood plasma. Potassium increased viability and speed of sperm movement at a concentration below that in the seminal plasma, whereas sodium and the nonelectrolytes were less effective. Potassium released with spermatozoa at spawning may therefore stimulate motility which has already been initiated by the decrease of osmolality.