Endocrine studies, using the four hypothalamic releasing hormones, gonadotrophin releasing hormone, thyrotrophin releasing hormone, corticotrophin releasing hormone and growth hormone releasing hormone, were performed in 33 males after admission with a major head injury. The test was repeated 7 days later in all 33 patients and after 3-6 months in 21 patients. All patients had a period of unconsciousness followed by post-traumatic amnesia of greater than 24 h. The results obtained from investigating the gonadal axis are reported in this paper. The levels of total and free testosterone, basal FSH and basal LH fell significantly during the first 3 days after injury, when the LH and FSH responses to GnRH achieved their highest peak levels. This hormone pattern has not previously been reported in the gonadal axis and may be a consequence of hypothalamic dysfunction. The severity of the injury was negatively correlated to the testosterone concentration on admission and to the basal and peak FSH concentrations 1 week later. Persistent hypogonadism was found in five of the 21 patients retested after 3-6 months, with low testosterone concentrations and three continued to have an exaggerated LH response to GnRH. Thus major head injury frequently results in hypogonadism shortly after injury, with an increased gonadotrophin response to GnRH. In addition, a substantial minority of patients continue to be hypogonadal after 3-6 months. In view of these findings we strongly suggest that all patients should be endocrinologically assessed at intervals following severe head injury.