Abstract Endplate potentials (epps) were recorded intracellularly from single diaphragm fibers of newborn (7–10 days, n=11) and older (24–30 days, n=11) rats in the presence of 100 nM ω-agatoxin IVA, a P-type Ca 2+ channel blocker. The muscle was stimulated via the phrenic nerve for 1 s at 40 Hz. In both age groups epp amplitude decreased with ω-agatoxin, however the decrease was greater in the older group (mean=60% of control vs. 40% of control in the younger group). A larger number of fibers in the older group (84% vs. 54% in the young) showed a≥50% decrease in epp amplitude. These data suggest that although P-type Ca 2+ channels are present in the immature presynaptic nerve terminals at the neuromuscular junction, functional maturation of these channels occurs with development. This may contribute to the susceptibility to neuromuscular transmission failure in the newborn diaphragm.