In 70 paired insulin tolerance tests (ITTs) and short 'Synacthen' tests (SSTs), both of which have been advocated for assessment of the hypothalamo-pituitary-adrenal (HPA) axis, there were 51 passes on both tests, 9 failures on both, and 10 discrepant results (9 failures on the SST, 1 failure on the ITT). There was a close correlation between the maximum cortisol value achieved in the two tests. A survey of British endocrinologists showed that only 24% used the SST to assess the HPA axis. It is suggested that the SST should be used for the initial assessment of the HPA axis and the ITT reserved for patients who fail the SST, those on corticosteroid therapy, and those who have had an acute pituitary insult within 14 days.