1. A method for adapting a standard stereotaxic frame for use with neonatal rats as young as postnatal day 1 (PD 1) was devised, and single-unit extracellular recordings were obtained from neurons in the locus ceruleus (LC) in urethan-anesthetized rats during different stages in development from PD 1 to PD 34. 2. The spontaneous firing pattern of neonatal LC neurons was characterized by long silent periods punctuated by brief epochs of sporadic firing. At PD 7-14, LC neurons exhibited periodic occurrences of irregular firing that lasted for 20-30 s. By PD 20, the pattern and rate of spontaneous activity were virtually indistinguishable from that of adults. 3. Conditioning stimulation of the dorsal noradrenergic bundle (DNB), given 10-200 ms prior to a test stimulus to the DNB, markedly reduced the amplitude of the antidromic action potentials to the test stimulus and sometimes resulted in spike decomposition. This refractoriness of the soma-dendritic membrane of LC cells was significantly attenuated with development and approached adult levels after PD 18. 4. Antidromic responses elicited by DNB stimulation were followed by a phase of inhibition or inhibition-excitation. Postactivation excitation was most prominent in cells that were not spontaneously active, and decreased steadily throughout development, probably because of the steady increase in spontaneous firing rate seen during maturation. 5. Although the conduction velocity of LC axons increased steadily from birth through PD 26, conduction time remained unchanged. 6. Neonatal LC neurons were equally sensitive to noxious and nonnoxious somatosensory stimuli. As development proceeded, LC neurons became less sensitive to innocuous somatosensory stimuli such as air puffs and tactile stimuli while simultaneously becoming more sensitive to noxious stimuli. Auditory and visual stimuli became effective for the first time at PD 14 and PD 12, respectively. 7. These results indicate that the electrical activity of LC neurons in the developing brain is intimately related to input from peripheral sensory sources. Therefore, the influence of the LC on the developing brain may occur predominantly through sensory input.