Six hospitalized volunteer male subjects were given insulin, 0.15 U/kg, before and after 14 days of administration of delta-9-tetrahydrocannabinol (THC) at a dose of 210 mg/day. A diminished maximal serum human growth hormone (GH) increase followed the prolonged THC ingestion. The mean maximal GH response was: 52.6 ng/ml +/- 8.7 (+/-SE) before THC and 18.8 ng/ml +/- 6.7 (+/-SE) during THC, P less than 0.01; corresponding cortisol responses were 20.1 mug/dl +/- 3.0 before THC and 10.0 mug/dl +/- 1.1 during THC, P less than 0.05. The data suggest suppression of the hypothalamic-pituitary axis after prolonged high dose THC use. This is consistent with other reported endocrine effects of marijuana in man.