In the present study we have examined the response of endogenous opiates (beta-EP and Met-enk) and ACTH to a particular type of thermal stimulus such as sauna in 8 young healthy subjects. Sauna-induced hyperthermia resulted in an increase of plasma beta-EP and ACTH, but appeared to have no significant effect on circulating Met-enk. The different responses of ACTH, beta-EP and Met-enk to heat exposure indicate that hyperthermia represents a form of stress, which can trigger off a well-defined neuroendocrine reaction.