Acetic acid (a strong irritant) applied to the hindlimb of frogs elicits vigorous wiping of the exposed skin; both pH and osmolarity may contribute to the nociception produced. This response is used in a standard test for analgesic effects in frogs, commonly termed the "acetic acid test". In this procedure, dilutions of the acid are placed drop-wise on the dorsum of the frog's thigh until the frog wipes the affected area.
