In this study, a rat model of inflammatory pain was produced by injecting complete Freund’s adjuvant into the hind paw, and the expression of acetylated histone 3 in the spinal cord dorsal horn was examined using immunohistochemical staining. One day following injection, there was a dramatic decrease in acetylated histone 3 expression in spinal cord dorsal horn neurons. However, on day 7, expression recovered in adjuvant-injected rats. While acetylated histone 3 labeling was present in dorsal horn neurons, it was more abundant in astrocytes and microglial cells. The recovery of acetylated histone 3 expression was associated with a shift in expression of the protein from neurons to glial cells. Morphine injection significantly upregulated the expression of acetylated histone 3 in spinal cord dorsal horn neurons and glial cells 1 day after injection, especially in astrocytes, preventing the transient downregulation. Our results indicate that inflammatory pain induces a transient downregulation of acetylated histone 3 in the spinal cord dorsal horn at an early stage following adjuvant injection, and that this effect can be reversed by morphine. Thus, the downregulation of acetylated histone 3 may be involved in the development of inflammatory pain.