Spironolactone, an aldosterone antagonist, was given in a daily dose of 100 mg to 15 patients with primary hypertension for one year. Fasting levels of lipids, uric acid, glucose, insulin, potassium and growth hormone were measured before and after 6 and 12 months of treatment. Total cholesterol, LDL cholesterol, glucose, potassium and growth hormone were unchanged, HDL cholesterol fell from (mean +/- SD) 1.5 +/- 0.6 to 1.1 +/- 0.3 mmol/l (p less than 0.05) after 6 months of treatment and remained lowered (1.0 +/- 0.3 mmol/l) (p less than 0.01) after 12 months of treatment. There was a transient fall after 6 months of treatment in triglycerides from 2.4 +/- 1.5 to 2.0 +/- 1.1 mmol/l (p less than 0.05), uric acid from 380 +/- 73 to 342 +/- 58 mumol/l (p less than 0.05) and an increase in insulin from 16 +/- 9.5 to 28.6 +/- 26.8 mU/l (p less than 0.05). The blood glucose curves above fasting levels after glucose loading were unchanged during spironolactone treatment, whereas the area under the net insulin curve was higher after 6 months of treatment (163 +/- 103 mU X h/l) than before treatment (105 +/- 71 mU X h/l), indicating a small and transient insulin resistance. Thus, spironolactone impaired the glucose tolerance transiently and gave small and almost transient changes in fasting serum lipid and uric acid levels.