Abstract Motility of bull spermatozoa depends to a great extent on the cationic composition of the extracellular medium. Potential difference and cellular membrane resistance were measured with microelectrodes in spermatozoa suspended in media of varying concentrations of potassium and sodium. The concentration gradient of potassium is a major determinant of the resting membrane potential, with sodium playing a minor role. With low (1 mM) or high (60 mM) potassium media effects were seen on velocity of the cell and amplitude of the flagellar wave as well as on bioelectrical activity. A minimal amount of sodium is necessary to maintain cellular motility.