The sensory innervation of the guinea-pig adrenal medulla was studied using the fluorescent retrograde tracer Fast Blue. Labelled neurons were found in the dorsal root ganglia at segments T3-L2, the greatest contribution arising from T10 representing 15.2% of the total number of labelled cells. Labelling was ipsilateral to the site of injection of tracer into the adrenal medulla and the labelled neurons ranged in size between 6 and 36 microns. The overall size distribution fell into two groups of between 6 and 15 microns and of between 18 and 36 microns. Presumed sensory nerve endings were found in the adrenal medulla associated with chromaffin cells and vascular elements. The nerve endings were ovoid to spherical and densely packed with mitochondria. It is concluded that events occurring in the guinea-pig adrenal medulla, and possibly cortex, are monitored by sensory nerve terminals whose cell bodies lie within dorsal root ganglia. The exact distribution of the sensory nerve terminals within the adrenal gland is yet to be determined.