Detailed endocrine studies were carried out in 95 hospitalized obese patients during their treatment with diet and the tricyclic anoretic mazindol. The results obtained after 1 week or more of mazindol (2 mg or occasionally 4 mg/day) administration were compared with the results after placebo and with the initial pre-treatment values. There were no significant changes in the following parameters: FSH, LH, testosterone, renin, angiotensin II, growth hormone (GH) levels during insulin tolerance tests (ITT), 131I uptake, basal metabolic rate, Achilles tendon reflexes, T3 RIA, rT3 RIA, 17-ketosteroids and 17-ketogenic steroids in urine, both basal and after stimulation with ACTH and metyrapone. Blood glucose and plasma immunoreactive insulin (IRI) levels during oral glucose tolerance tests decreased during mazindol administration, IRI levels were significantly lower during ITTs after mazindol. T4 RIA serum levels increased significantly after mazindol. When mazindol was administered, GH levels increased somewhat in some obese patients during ITTs, while T3 RIA and rT3 RIA decreased in some patients. Mazindol has not only hunger (appetite) suppressing properties, but it probably affects the metabolism of energy substrates as well. The drug was well tolerated and there were no pathological findings in routine laboratory examinations during a long-term study with mazindol (non-stop treatment for 6 months).