Diltiazem is a calcium channel blocker with antihypertensive and antiarrhythmic actions. In the present study 24 hypertensive patients were treated with diltiazem (180-360 mg daily) for 6 months. The effects on the electrocardiogram (ECG) were studied and were related to the serum levels of diltiazem. Treatment with diltiazem induced a non-significant reduction in heart rate with 3 beats min-1 and decreased blood pressure (-11/-9 mm Hg, p < 0.001). The treatment also induced a prolongation of the PQ-interval (+1.4 cs, p < 0.02), a prolongation only being weakly related to the plasma levels of diltiazem (r = 0.28, not significant) and its metabolite MA (r = 0.37, p = 0.07). Neither was the induced increase in the QT-interval (+0.9 cs, p < 0.05), significantly related to the plasma levels of diltiazem. In conclusion, the plasma levels of diltiazem were not clearly related to the induced changes at the ECG during antihypertensive treatment.