Left ventricular function was investigated by radionuclide ventriculography in 13 patients (11 male, two female) with hypertrophic cardiomyopathy, aged from 22-57 years (mean 45.5 years) at rest and during exercise. Ten patients had hypertrophic obstructive cardiomyopathy with maximal left ventricular outflow tract gradients of 64-290 mmHg (mean 147 mmHg). Left ventricular enddiastolic pressure of all patients ranged from 8-35 mmHg (mean 21 mmHg). Radionuclide ventriculography was performed without therapy, after acute application of a single oral dose of gallopamil (50 mg), and after longterm treatment for 3 weeks (50 mg tid). Ejection fraction at rest after single dose increased from 69.2% to 72.9% (p less than 0.02), peak ejection rate (PER) increased from 333.5 to 362.0/s (p less than 0.01) and peak filling rate (PFR) from 284.5 to 316.5/s (p less than 0.02). Under exercise single dose as well as longterm treatment led to a slight but significant shift in the ratio of PFR/PER (from 1.02 to 1.12 after single dose [p less than 0.04], and to 1.18 with longterm treatment [p less than 0.03]). There was no correlation between the individual response to gallopamil treatment and histopathological parameters such as hypertrophy or fibrosis. These data demonstrate that gallopamil in patients with hypertrophic cardiomyopathy leads to an improvement mainly in left ventricular diastolic function which appears to be most effective under exercise.