Rats, from birth to postnatal day 34, were anesthetized with urethane and a neuropharmacological study was carried out of the autoreceptors located on the somadendritic membranes of locus coeruleus (LC) neurons. Iontophoretic application of noradrenaline (NA) caused inhibition of LC cell firing at all developmental stages, and such inhibition was totally blocked by the alpha 2-antagonist piperoxane. The sensitivity of LC neurons to iontophoretically applied NA appeared to become reduced with age. In LC neurons from birth to postnatal day (PD) 8, the prolonged period of suppressed firing after antidromic activation by stimulation of the dorsal noradrenergic bundle was not shortened by piperoxane. After PD 9, the proportion of LC neurons in which piperoxane could antagonize the postactivation inhibition increased with age. These results indicated that although LC neurons, even at birth, had alpha 2-adrenoceptors on the somadendritic membranes which were responsible for the NA-induced inhibition, inhibition of LC cell firing caused by NA released from the terminals of axon collaterals and/or possibly from dendrodendritic synapses did not occur until PD 9.