It remains difficult to make a reasoned choice between betablockers and calcium antagonists in the medical treatment of hypertrophic cardiomyopathy. In order to help in making this choice, we compared the effects of 320 mg of propranolol and 480 mg of verapamil, prescribed in a random order for an average period of 2.5 months in 24 patients. In 15 patients the two therapeutic sequences were preceded and followed by a clinical examination and an exercise stress test after withdrawal of all treatment. The overall functional status correlated to oxygen consumption was unchanged by the two drugs, but patients felt better more often with verapamil. These was no significant change in maximal power developed during exercise testing although verapamil increased it by 9 p. 100 throughout with a corresponding increase in oxygen consumption. Two parameters were significantly changed by both drugs: the maximal heart rate fell from 161 +/- 22 to 122 +/- 18 with propranolol, and to 145 +/- 28 with verapamil; the oxygen pulse (equivalent to the systolic index) increased from 10.9 +/- 2.2 to 14.8 +/- 3.9 with propranolol, and to 13.7 +/- 2.4 with verapamil. There were no changes in the blood pressure profiles during exercise. Side effects were observed with both drugs; muscular weakness was the biggest problem with propranolol and sinus node dysfunction with verapamil.(ABSTRACT TRUNCATED AT 250 WORDS)