To evaluate the effects of long-term treatment antihypertensive with the dihydropyridine calcium antagonist amlodipine on insulin sensitivity, plasma insulin, and lipoprotein metabolism in obese hypertensive patients. We measured the insulin sensitivity index (SI), determined by the Minimal Model Method of Bergman, fasting plasma insulin and glucose concentrations, serum total triglyceride and lipoprotein cholesterol fractions, and blood pressure in 20 obese, non-diabetic patients with essential hypertension before and after 6 weeks of placebo and again after 6 months of amlodipine. Ten patients [mean body mass index (BMI) 30.2 kg.m-2] had been on prior treatment with a thiazide diuretic in low dosage and/or a beta-adrenoceptor blocker (group A), and 10 matched patients [BMI 31.8 kg.m-2] had been previously untreated (group B). Amlodipine was started in a dose of 5 mg and was increased to 10 mg once daily in 14 patients who were hypertensive after 8 weeks on the lower dosage. At entry (before placebo), SI was slightly but not significantly lower in group A than B [2.7 vs. 3.6 x 10(-4) ml.microU-4.min-1]; fasting plasma insulin was 13.6 vs. 12.9 microU.ml-1. After 6 weeks on placebo, S1 averaged 3.7 in group A and 4.4 x 10(-4) microU.ml-1.min-1 in group B; fasting plasma insulin was 14.6 vs. 15.1 microU.ml-1, and glucose 5.5 vs. 5.5 mmol.l-1.(ABSTRACT TRUNCATED AT 250 WORDS)