The formalin assay is the most popular chemical assay of nociception. It entails the injection of a dilute solution of formalin into the surface of the rodent's hindpaw, followed by the scoring of stereotypical behaviors such as flinching, licking, and biting of the affected hindpaw.[1] The behaviors last for approximately 1 hour, with the early or acute stage (directly after injection) reflecting direct activation of nociceptors and the late or tonic phase (15 to 20 minutes after the injection) reflecting inflammation.[1] Typically, the formalin assay is used on rats; however, formalin concentrations and scoring methods can be modified as to suit mice.[2] One major advantage of the formalin assay over other models of inflammatory pain is the limited duration (approximately 1 hour) of the response.[2] Additionally, as described before, this assay produces a response in two discrete stages, allowing researchers to model both acute and tonic pain using a single noxious chemical.