The role of electrical activity in the developmental regulation of cholinergic neurons was investigated in dissociated spinal cord--dorsal root ganglion (SC-DRG) cultures. Application of tetrodotoxin (TTX) during the first 6 days after plating had no effect on choline acetyltransferase (CAT) activity. Suppression of electrical activity during the 7th day decreased CAT to 68% of control. These decreases in CAT activity were still apparent 2 weeks after removal of the TTX. GABAergic neurons, as indicated by glutamic acid decarboxylase activity and high affinity [3H]GABA uptake, were not affected by TTX treatment. Addition of 8-bromo-cAMP or conditioned medium obtained from SC-DRG cultures at certain developmental periods produced dose-dependent increases in CAT levels on TTX-treated cultures as compared with those treated with TTX alone. Similar studies with 8-bromo-cGMP revealed no significant effects on CAT activity. Vasoactive intestinal peptide (VIP) produced a dose-dependent increase in CAT activity when added to cultures between days 12 and 14. Similar studies conducted on younger cultures (days 5-7) or older cultures (days 21-23) revealed no increases in CAT activity. Addition of 0.1 nM VIP to TTX-treated cultures resulted in CAT levels which were not significantly different from those of electrically active controls. These data suggest that cyclic AMP, VIP, and trophic substances in conditioned medium may have roles in the mechanism of cholinergic toxicity produced by electrical blockade of developing spinal cord neurons.