Mast cell density is increased in the lumbar dura mater at 24 hours and 72 hours post-carrageenan in female rats. Intraplantar carrageenan does not induce a significant increase in percent degranulation or any changes in total, granulated, or degranulated cell densities in male rats in the lumbar (A, E) or in thoracic (B, G) dura mater. There is no significant increase in percent mast cell degranulation after carrageenan treatment in female rats in the lumbar dura mater (B) or in the thoracic dura mater (D). However, intraplantar carrageenan induces a significant increase in total, granulated and degranulated mast cell densities in the lumbar dura mater at 24 hours post-injection and in degranulated and total mast cell densities at 72 hours post-injection (* = P < 0.05, ** = P < 0.001) (F). 1% carrageenan also induces increased total, granulated and degranulated mast cell densities at 24 hours post-injection in the lumbar dura mater (F). Degranulated cell density is increased in the thoracic dura mater of female rats at 72 hours post-injection (H).