Neurotransmitter inhibition of calcium currents (ICa) can be relieved by large depolarizing prepulses. This effect has been postulated to be due either to the voltage-dependent unbinding of an inhibitory molecule from the channel or to a slow voltage-dependent gating step intrinsic to the modulated channel. According to the first hypothesis, the rate of reinhibition (reblock) following a depolarizing prepulse should depend on the concentration of active inhibitory molecules and thus should increase with the extent of inhibition. To distinguish between these models we examined the actions of norepinephrine (NE) and somatostatin (SS) on high-threshold calcium currents in chick sympathetic ganglia, using whole-cell voltage-clamp methods. As previously described in other systems, both NE and SS inhibit omega- conotoxin-sensitive N-type Ca2+ current in a voltage-dependent manner. Pertussis toxin (PTX) pretreatment prevents the inhibition of the current, while replacing GTP in the patch pipette with GTP-gamma-S results in irreversible inhibition, consistent with the involvement of a PTX-sensitive G-protein. The inhibitory responses to NE and SS are not additive, suggesting that they act at a common locus. The inhibitory response to repeated applications of NE or SS desensitizes, with little evidence for cross desensitization. The inhibition of ICa is relieved by a 15 msec prepulse to +100 mV. Following repolarization to -80 mV, ICa slowly reblocks. During prolonged applications of NE or SS the extent of inhibition decreases due to desensitization and reblock kinetics are significantly slowed (time constant increases from 60 msec to > 100 msec for both NE and SS). These results are well fit by a quantitative model in which the kinetics of reblock reflect the binding of an inhibitory molecule to the channel.