This study investigated the effects of different environmental conditions on the motility parameters of paddlefish (Polyodon spathula) spermatozoa. Paddlefish spermatozoa demonstrated the following characteristics: (i) all spermatozoa were motile 10 s after activation with a velocity of 130-160 microm s(-1); (ii) after 2 min, velocity decreased to 80-130 microm s(-1); and (iii) motility was maintained for up to 9 min. Concentrations of 0.5-5.0 mmol KCl l(-1) prevented activation of spermatozoa. After transfer into a swimming medium (20 mmol Tris l(-1), pH 8.2 and 1 mg BSA ml(-1)) containing 0.5 mmol KCl l(-1) (combined with 5 mmol NaCl or MgCl(2) l(-1)), 80-100% of cells were motile with a velocity of about 120-150 microm s(-1). MgCl(2) significantly improved the velocity of spermatozoa at 10, 40, 50 and 60 s after activation and the stable velocity of spermatozoa was about 140 microm s(-1). Very low concentrations of CaCl(2) (0.125 mmol l(-1)) combined with 0.5 mmol KCl l(-1) initiated motility in 20% of spermatozoa, whereas all spermatozoa were activated after 2 min with 0.25 mmol CaCl(2) l(-1) in similar medium for the full period of swimming with velocity of about 120 microm s(-1). This study demonstrated that potassium (5-15 mmol l(-1)) inhibits demembranated spermatozoa. Thus, initiation of movement in paddlefish spermatozoa is under the reciprocal control of potassium and calcium ion concentrations.