The prepacemaker nucleus (PPN) in the midbrain of the gymnotiform electric fish Eigenmannia provides the only known neuronal input to the medullary pacemaker nucleus, which triggers each electric organ discharge (EOD) cycle by a single command pulse. Electrical stimulation of the PPN elicited two distinct forms of modulations in the pacemaker activity, brief accelerations, hence referred to as 'chirps', and gradual frequency shifts with a time constant of approximately one second. The associated EOD modulations were indistinguishable from natural communication signals. Depending upon the site of stimulation, the two forms of modulation could be elicited alone or superimposed (Fig. 1). Stimulation sites eliciting only chirps could be separated from sites eliciting only gradual shifts by as little as 60 micron. The magnitude of the elicited chirps depended upon the timing of the pulse stimulus with reference to the phase of the pacemaker cycle (Figs. 2, 3). Extracellular and intracellular recordings of single PPN neurons revealed that an action potential from a single neuron generates a chirp, and that the magnitude of the chirp depends upon the timing of the action potential with reference to the phase of the pacemaker cycle (Figs. 4, 5). The spike activity of these neurons had no relation to the jamming avoidance response (JAR), suggesting independent neuronal mechanisms for chirps and the JAR. Depolarization of such neurons by current injection produced bursts of chirps (Fig. 6), and intracellular injection of Lucifer Yellow identified these cells as a large type of PPN neuron which could also be retrogradely labeled from the pacemaker with horseradish peroxidase (HRP) (Fig. 7).(ABSTRACT TRUNCATED AT 250 WORDS)