1. Plasma adrenocorticotrophic hormone (ACTH) concentrations were measured in rats following exposure to anaesthetic agents, after stimulation of peripheral sensory nerves, and during psychological stress. 2. In rats, kept in their home cages, the i.p. injection of sodium pentobarbitone did not cause an increase in plasma ACTH, whereas injection of urethane increased plasma ACTH several times. In rats transferred to a glass dessicator and inhaling oxygen, plasma ACTH was more than 3 fold higher than in rats in their home cage. Exposure to nitrous oxide, halothane or ether in a glass dessicator produced significantly higher plasma ACTH concentrations when compared to exposure in the home cage. 3. In rats anaesthetized with pentobarbitone, the electrical stimulation of large myelinated afferents in the sciatic nerve did not trigger a measurable increase in ACTH secretion, whereas stimulation of afferent A delta- and C-fibres significantly elevated plasma ACTH concentrations. Rats treated as neonates with capsaicin showed an attenuated ACTH response to A and C-fibre stimulation. 4. Similarly, capsaicin pretreatment reduced the increase in ACTH secretion during morphine withdrawal; a similar effect was produced by clonidine. 5. ACTH secretion following open field exposure, ether stress or hypoglycaemia was not changed by capsaicin pretreatment. 6. It was concluded that capsaicin-sensitive afferents are involved in the secretion of ACTH elicited by somatosensory forms of stress. Centrally evoked ACTH release is not affected by capsaicin pretreatment.