1. Calcium channel blockers were tested on excitatory postsynaptic currents (EPSCs) at the synapse formed by the calyx of Held on the principal cells in the medial nucleus of trapezoid body (MNTB) in brainstem slices of 4- to 14-day-old rats. 2. At postnatal day 4-9 (P4-9), EPSCs were irreversibly suppressed by the P/Q-type Ca2+ channel blocker omega-agatoxin-IVA (omega-Aga-IVA, 200 nM) and also by the N-type Ca2+ channel blocker omega-conotoxin GVIA (omega-CgTx, 2 microM). A small fraction of EPSCs was resistant to both toxins but abolished by Cd2+ (100 microM). 3. After P7, the omega-CgTx-sensitive EPSC fraction diminished and eventually disappeared after P10. Concomitantly the fraction insensitive to both toxins decreased and became undetectable after P10. 4. In contrast, the omega-Aga-IVA-sensitive EPSC fraction increased with development and became predominant after P10. All through the developmental period examined, the L-type Ca2+ channel blocker nicardipine (10 microM) had no effect. 5. We conclude that presynaptic Ca2+ channel types triggering transmitter release undergo developmental switching during the early postnatal period.