1. Calcium currents and transmitter release were studied in cocultures of fetal mouse neurons from the ventral half of the spinal cord (VH neurons) and from dorsal root ganglion (DRG neurons). The effects of BayK 8644 and omega-conotoxin on calcium currents and transmitter release were compared. 2. The presence of low voltage-activated (LVA) calcium current in both VH and DRG neurons is variable. Some cells exhibit only high voltage-activated (HVA) currents, whereas others show both HVA and LVA currents. 3. BayK 8644 did not affect LVA currents but strongly augmented both steady and transient components of the HVA calcium conductance. 4. omega-Conotoxin GVIA reduces both transient and steady components of the HVA but does not abolish either component even after 3 h of application. 5. Calcium currents that were resistant to omega-contoxin were augmented by BayK 8644. 6. Synaptic transmission between pairs of spinal cord neurons from the ventral half of the spinal cord (VH-VH connections) or between dorsal root ganglion neurons and VH neurons (DRG-VH connections) were studied with two-cell recording and stimulation techniques. 7. In approximately 70% of VH-VH connections and 50% of DRG-VH connections, BayK 8644 or its active optical isomer failed to affect transmitter output. Substantial augmentation of the remainder of the connections could be reliably produced by the dihydropyridines. Raised calcium in the extracellular medium produced augmentation of synaptic connections in all cases. BayK 8644 produced substantial, consistent augmentation of voltage-sensitive calcium channels in both VH and DRG neurons. 8. The toxin, omega-conotoxin, produced no consistent effect on excitatory or inhibitory postsynaptic potentials (EPSPs or IPSPs) elicited in VH neurons by stimulation of nearby VH neurons. VH EPSPs elicited by stimulation of nearby DRG neurons were reduced to approximately 50% of control values after 10 min of omega-conotoxin perfusion. Spontaneous and evoked synaptic activity could be recorded in VH neurons as long as 2 h after cultures were incubated in 0.5 microM omega-conotoxin. omega-Conotoxin produced a modest reduction in HVA currents in both VH and DRG neurons. 9. BayK 8644 did not produce consistent augmentation of transmission at the frog neuromuscular junction. omega-Conotoxin produced total blockade of transmission in this preparation. 10. We conclude that neither sustained nor inactivating high-threshold voltage-sensitive (HVA) calcium channels sensitive to BayK 8644 or omega-conotoxin such as those measured in the neuronal cell bodies are responsible for action-potential-evoked transmitter release from the majority of VH neurons.(ABSTRACT TRUNCATED AT 400 WORDS)