Three calcium antagonists, diltiazem, flunarizine and verapamil stimulated human sperm motility in vitro. Among them, diltiazem induced the largest amplitude of motility increase. The concentration-response curve of diltiazem was similar in shape to those of calcium chelators. EGTA, a calcium chelator, potentiated the stimulatory effect of diltiazem while A23187, a calcium ionophore, antagonized it. These observations supported our previous hypothesis that an increased concentration of calcium ion was detrimental to human sperm motility. Whether calcium antagonists could be used in the treatment of subfertile patients awaits further investigation.