Cyclic nucleotides are believed to mediate a long-lasting synaptic hyperpolarization in the bursting pacemaker neuron, R15, and are capable of inducing bursting pacemaker activity in the usually silent metacerebral giant cell. Steady state voltage clamp techniques were used to examine the alterations of membrane characteristics produced in these different cell types by cyclic nucleotides. In both cells, IBMX, a phosphodiesterase inhibitor, increased two components: (1) voltage-dependent sodium current and (2) slope conductance believed to reflect potassium flux. The effects of 8-benzylthio-cAMP were identical to those of IBMX in the metacerebral cell. In R15, 8-benzylthio-cAMP affected only the slope conductance. These results are discussed in terms of cyclic nucleotide of bursting pacemaker activity.