"Depolarization-dependent calcium influx is biphasic in synaptosomes (Nachshen and Blaustein, 1980, 1982). The fast and slow components of 45 Ca++ influx are proposed to occur through separate calcium channels. In this report the fast and slow components of depolarization-dependent 45Ca++ influx are resolved. The fast component is inactivated by predepolarization, inhibited by low concentrations of lanthanum and closely correlates with the rapid release of 3H-norepinephrine from synaptosomes. The properties of the fast component of influx suggest that it is mediated by calcium channels which inactivate during the first second of depolarization. The slow component is not sensitive of predepolarization and is partially dependent on the internal Na+ content of the synaptosomes. It is proposed that during depolarization a portion of the slow component of Ca++ influx in synaptosomes is mediated by a Na i/Cao exchange mechanism."