The effect of injection of ACTH 1-17 on the secretion of GH has been examined at three different circadian stages (07, 14 and 21 h) in the winter. Each week for six consecutive weeks the subjects were investigated before and after I.M. injection either of saline (controls) or 100 micrograms ACTH 1-17. Each subject was his own control. The peak plasma concentration of GH was always found 40 min after the injection. The larger absolute increase and the greater AUC curve were seen following the injections at 14.00 h and 21.00 h. The findings could not be attributed to an effect on GH-RH, since secretion of it remained unchanged during the test.