Aims: To investigate whether exogenously administered 5-hydroxy-tryptamine (5-HT) at high or low concentration influences pain and microcirculation in the human masseter muscle. Methods: In 12 healthy female subjects, 5-HT in 2 concentrations (0.1 µmol/L and 1,000 µmol/L) and isotonic saline were injected into the masseter muscles in a randomized and balanced double-blind manner. The pain intensity after injections was recorded with Borg's rating scale, and intramuscular blood flow was monitored continuously during the experiment with a laser-Doppler technique. Nonparametric statistics were used for analyses. Results: Administration of 5-HT at 1,000 µmol/L induced significantly more pain than saline (Wilcoxon: P .05), while there was no difference between 5-HT at 0.1 µmol/L and saline. The blood flow did not change significantly after injection of 5-HT at either concentration compared to saline. However, changes in pain intensity and blood flow were positively correlated after injection of 5-HT at 1,000 µmol/L (Spearman: P .05). Conclusion: Intramuscular administration of 5-HT at 1,000 µmol/L into the human masseter muscle induced pain, but 5-HT did not have any effect on local blood flow at either concentration. Keywords: ischemia, laser-Doppler flowmetry, masseter muscle, pain, serotonin