This study characterizes the inhibiting effect of diltiazem and nitroglycerin on periodic contractions of isolated human coronary arteries. Isometric force of coronary ring segments from sixty-nine cadavers was recorded in a muscle bath. To quantify the experimental results, we used 3 X 10(-6) M prostaglandin F2 alpha to induce the periodic contractions of a certain force. When diltiazem was added during the periodic contractions, the amplitude of oscillations gradually decreased until eventually oscillations ceased completely. The process prior to the cessation of the oscillations was characterized mainly by the inhibition of the contraction phase. The inhibition rate at the time of the complete cessation of oscillations was 49.3 +/- 6.3% at 5 X 10(-7) M. The time required for complete disappearance of oscillations was dependent on the diltiazem concentration. When nitroglycerin was added during periodic contractions, the oscillations did not disappear. Compared to the contraction phase, the relaxation phase was appreciably inhibited. With only 10(-7) M diltiazem, the rate of inhibition of the contraction phase was 22.0 +/- 7.7%, whereas the preliminary treatment with 5 X 10(-8) M nitroglycerin led to a complete cessation of the oscillations, and suppression of the level of the contractions to a significantly greater extent, viz. 58.7 +/- 5.8% (p less than 0.001). Therefore, it is considered more effective in the treatment and prevention of coronary spasm to use diltiazem and nitroglycerin simultaneously rather than individually.