A golgi-Cox examination of the locus coeruleus in the spontaneously hypertensive rat (SHR) has shown an increase in the number of primary dendritic branches, in the number of secondary branch points, in the mean length of the dendritic branches from the soma to the outer extent of the secondary branches, and in the longest extent of the dendritic domain in comparison with the locus coeruleus in the Wistar-Kyoto normotensive rats at sixteen weeks of age. We suggest that the anatomical changes may provide a substrate for altered afferent relationships in the neurochemical regulation of neurons of the locus coeruleus, although it remains to be shown whether such alterations are related in any way to blood pressure. It appears evident that the altered mechanisms resulting in hypertension in this genetic model are more complex than an anatomical or neurochemical modification in one of the central autonomic nuclei. However, the present finding that anatomical alterations in the dendritic arborizations of a key nucleus such as the locus coeruleus occur, points towards a need for further examination of the interrelationships of specific neurotransmitter systems in this nucleus.