Repeated intramuscular injections of acidic saline in rats are reported to induce a chronic type of muscle sensitization without causing significant tissue damage in this model two repeated injections of acidic saline (pH 4) into the gastrocnemius muscle produce short-lasting pain, but allodynia that lasts for 4 week with pain spread to the contralateral side. This may indicate that central sensitization occurs and this model is thus believed to more accurately mimic chronic myalgia. Our aim was to find out if repeated injection of buffered acidic saline into the masseter muscle causes pain and hyperalgesia and if so this would be a valid experimental model for orofacial myalgia / myofascial TMD in humans. Twenty healthy and pain-free subjects (10 male + 10 female: 25.1 ± 0.9 years) were included. Pain levels were assessed on a 0-10 numerical rating scale (NRS) and pressure pain thresholds (PPTs) were recorded using an electronic pressure algometer. The subjects received a unilateral intramuscular injection (0.5 ml) of acidic saline (pH 3) or placebo (isotonic saline, pH 6) into the masseter muscle (randomized and double-blind). Two days thereafter the injection was repeated with the same substance. Pain was assessed 5 min before the injection and at 5, 15, 30, 45 min, 1, 3, 24 hours after both injections and then at 4 and 7 days after the second injection and PPTs at 5, 15, 30, 45 min, 1 hrs after both injections and at 1, 4 and 7 days after the second injection. The experiment was repeated with the other substance after at least one week. There was no difference between substances in NRS pain scores after the injections. One way ANOVA indicated a time effect but no substance effect on PPTs. These preliminary results do not support acidic saline injections as a valid pain model for experimental masseter myalgia in healthy subjects.