UNLABELLED: The aim of this study was to determine the relationships between plasma free fatty acids levels (FFA) and impaired glucose tolerance in hyperthyroid patients in basal condition and after propranolol and acebutolol treatment, i.e. after lipolytic blockade leading to a decrease in plasma FFA levels. 31 untreated hyperthyroid patients and 12 normal subjects of matched control group were studied. In all IVGTT was performed. 19 patients with normal K values were randomized to treated with either propranolol 160 mg daily (10 persons) or acebutolol 400 mg daily (9 persons). 12 patients with low K values were randomized to treated with either propranolol (5 persons) or acebutolol (7 persons). After 7 days on treatment a second IVGTT was carried. In all blood samples were glucose, FFA and insulin levels determined, in samples taken at 0 T3 and T4 too. T3, T4 and insulin were measured by the radioimmunoassay, glucose by the glucose oxidase method, FFA by the colorimetric method. In hyperthyroid patients with low K value significantly higher plasma concentrations of FFA were found compared to control and after propranolol or acebutolol treatment no differences in FFA levels between hyperthyroid patients and control were observed. In patients with low K value acebutolol but not propranolol normalized K value and propranolol but not acebutolol significantly decreased acute phase of insulin secretion. In patients with normal K value plasma glucose and insulin concentrations as well K value were not altered during propranolol or acebutolol treatment. CONCLUSIONS: 1. Hyperthyroid patients with low K value differ from healthy subjects higher plasma FFA levels, 2.(ABSTRACT TRUNCATED AT 250 WORDS)