Objectives: To determine the in vitro effects of cocaine on sperm motility and bovine mucus penetration because cocaine abuse is associated with decreased sperm motility, and related compounds, such as procaine, are known to decrease sperm motility. Design: Human semen samples were exposed to a range of cocaine concentrations and the effects quantified using computer- assisted sperm analysis and the bovine mucus penetration test. Setting: University research laboratory. Patients, Participants: Samples were obtained from 18 healthy volunteers. Interventions: Normal semen samples were exposed to concentrations of cocaine ranging from 10-11 to 10-4 M. Motility characteristics were evaluated after 2 hours, and bovine mucus penetration was evaluated after 30 minutes, 1 hour, and 2 hours. Mucus penetration by washed sperm was also evaluated. Main Outcome Measures: Motility characteristics were evaluated using computer-assisted sperm analysis, and functional sperm motility was evaluated using the bovine mucus penetration test. Results: Cocaine exposure decreased the percentage of motile sperm in a concentration-dependent manner with a maximum decrease of 23% at 10-4 M but had no effect on other motility characteristics. Cocaine decreased bovine mucus penetration by 12% at high cocaine concentrations (10-4 M), but increased penetration by 69% at low concentrations (10-9 M). Washing sperm before cocaine exposure attenuated the increased sperm penetration. Conclusion: The ability of cocaine to decrease the percentage of motile sperm at high concentrations may explain the decreased sperm motility associated with cocaine use. Cocaine's ability to augment sperm penetration at low concentrations suggests an interaction of cocaine with the sperm adrenergic system.