In this study, IP administration of acetaminophen and its half-life were investigated. Acetaminophen as an analgesic drug ordinarily is used in tablets or syrup form. Therefore, there is no report about administration of acetaminophen through injection. Acetaminophen in ED50 dose (200 mg/kg) was injected into the animals intraperitoneally, and its analgesic effect was investigated by formalin test. We used acetaminophen 10 and 30 min before and at the same time of formalin injection. Our results showed that the usage of acetaminophen 10 min prior to formalin injection could exert a significant analgesic effect in chronic phase of formalin test. Also, a marked analgesic effect was obtained in the acute and middle part of chronic phase (15-40 min) of formalin test. However, the late phase (35-60) of chronic pain was diminished significantly, when acetaminophen administered at the same time of formalin injection. In addition, our data show that acetaminophen can be used as injection form, and through IP route has a half-life of nearly 1 hour.