SUMMARYThe interaction between the mechanisms involved in the LH, FSH, growth hormone, prolactin, ACTH and TSH responses to the synthetic LH and FSH releasing hormone, thyrotrophin releasing hormone and insulin induced hypoglycaemia was studied in twelve normal male volunteers. Each subject acted as his own control and the test procedures were performed individually and in combination. The simultaneous administration of one releasing hormone with another or with insulin in no way modified the hormonal responses to either releasing hormone or to hypoglycaemia. Clinical testing with these procedures may therefore be performed simultaneously, so that the pituitary reserve for the five anterior pituitary hormones may be assessed together in under 2 hr. In addition it has been shown that TRH releases a small amount of FSH but not LH in male subjects.