It has been previously reported that sauna-induced fevers (approximately 39 degrees C) result in rises of beta-endorphins in normal volunteers. This report describes changes in plasma beta-endorphins in cancer patients undergoing whole body hyperthermia (40.5 degrees C to 41.8 degrees C). Results presented show that there is a linear relationship between thermal stress, defined in terms of core temperature and/or duration of hyperthermia, and the quantitative rise in plasma beta-endorphin levels. Data relating to changes in ACTH and cortisol levels are in a single temperature range (41.5 degrees C--41.8 degrees C) are also reported.