The locus coeruleus of the adult albino rat is a clearly delimited nucleus in Nissl-stained preparations. It is surrounded by an extensive, relatively neuron-free neuropil which is not stained in reduced silver and Luxol fast blue preparations. Most if not all locus coeruleus neurons contain the enzyme dopamine-beta-hydroxylase (DBH) and are thus presumably adrenergic. Two general classes of medium-sized neuron were found in the locus coeruleus in Nissl- and DBH-stained material: multipolar and somewhat smaller fusiform cells. The nucleus was divided into dorsal and ventral parts cytoarchitectonically; the two are distinguished in that (a) fusiform rather than multipolar cells predominate in the dorsal division, (b) cells in the dorsal division are more densely packed, and (c) a majority of the cells in the dorsal division are aligned obliquely in a dorsolateral to ventromedial orientation when viewed in the frontal plane, and longitudinally (anteroposteriorly) when viewed in the horizontal and sagittal planes. The locus coeruleus contains an estimated 1643 +/- 21 neurons (+/- S.E.M.; N=12) as determined in Nissl-stained paraffin sections, and 1439 +/- 29 neurons (+/- S.E.M.; N=6) as determined in DBH-stained frozen sections. The latter estimate is less reliable because of some uncertainty about section thickness. The ventral division of the locus coeruleus has an estimated 210 +/- 11 neurons (+/- S.E.M.; N=6). In Golgi-Cox material counterstained with cresyl violet most locus coeruleus neurons could also be classified as multipolar or fusiform, the latter being somewhat smaller. Typically, both types of neuron have relatively long thin dendrites which branch once or twice and extend well beyond the limits of the nucleus into surrounding neuropil and nuclear areas, particularly the mesencephalic nucleus of the trigeminal nerve and pontine central gray. Spines, consisting of a thin stalk of variable length with a small bulb at the end or just a thin stalk, were scattered infrequently but regularly along all dendrites and a majority of the somata of both classes. Very thin locally ramifying axon-like plexuses were impregnated in several locus coeruleus neurons, as were larger (about 3 mum diameter) projecting axons. Only about 0.07% of the locus coeruleus neurons were impregnated in the Golgi-Cox material although a wide range of impregnation times and ages was used.