Depolarization-induced (potassium-stimulated) influx of 45Ca, 85Sr, and 133Ba was measured in synaptosomes prepared from rat brain. There are two phases of divalent cation entry, "fast" and "slow;" each phase is mediated by channels with distinctive characteristics. The fast channels inactivate (within 1 s) and are blocked by low concentrations (less than 1 micro M) of La. The slow channels do not inactivate (within 10 s), and are blocked by high concentrations (greater than 50 micro M) of La. Divalent cation influx through both channels saturates with increasing concentrations of permeant divalent cation; in addition, each permeant divalent cation species competitively blocks the influx of other permeant species. These results are consistent with the presence of "binding sites" for divalent cations in the fast and slow channels. The Ca:Sr:Ba permeability ratio, determined by measuring the influx of all three species in triple-label experiments, was 6:3:2 for the fast channel and 6:3:1 for the slow channel. A simple model for ion selectivity, based on the presence of a binding site in the channel, could account well for slow and, to some extent, for fast, channel selectivity data.