In order to pursue the significance of the ionic regulation of sea urchin sperm behaviour, alterations in the cation composition of sea water were tested for their effects on sperm fertilizing capacity. Nearly all changes which resulted in lowered sperm intracellular pH, including lowered sea-water pH, inclusion of the divalent ion chelator EGTA, addition of dithiothreitol, or removal of sea-water Na+, enhanced sperm viability for periods of up to a week. These conditions caused decreased cell motility and elevated ATP concentrations, and prevented the acrosome reaction. Conversely, changes which increased the intracellular pH, decreased sperm ATP concentrations, or induced the acrosome reaction, reduced sperm viability. A single medium, high sea-water K+ concentrations (greater than 100 mM), provided an exception to these general trends. At elevated K+ concentrations sperm were quiescent but became completely infertile. These data show that sperm fertilizing capacity is generally extended by maintenance of the sperm in an inactive state, and the results suggest that decreased cellular energy levels contribute to decreased fertility.