(A) In the normal (unprimed) paw, a small intradermal injection of prostaglandin E2 (PGE2) causes an episode of acute hyperalgesia (decreased threshold for paw withdrawal from a pressure stimulus) lasting less than 4 hours. (B) Injection of the inflammogen, carrageenen, causes an episode of hyperalgesia that lasts less than 4 days (gray-filled curve). After carrageenan-induced hyperalgesia is no longer present, the paw remains in a latent state of hyperalgesic priming. In this state, an injection of PGE2, which would cause only acute hyperalgesia in the normal (unprimed) paw, now induces an additional chronic hyperalgesia. In comparison to the unprimed paw, this hyperalgesia is greater in magnitude and is greatly prolonged, lasting at least 3 weeks.