During an investigation of the possible pharmacokinetic interactions of cyclooxygenase-2 (COX-2) inhibitors with metoprolol, we observed that rofecoxib caused a significant reduction in heart rate in young healthy volunteers. The effect of valdecoxib did not reach significance. When these drugs were given together with metoprolol, the effect was continued. The latter effect could not be related to pharmacokinetic interactions. In the light of experimental results claiming a cardioprotective effect of possibly COX-2-derived prostaglandins and clinical observations hinting at an increased risk of sudden cardiovascular death in conjunction with the long-term use of selective cyclooxygenase inhibitors, our results may help to increase awareness and to suggest investigation of the impact of coxibs on heart function.