We examined the production of PAF, a mediator of shock, and LysoPAF, an inactive metabolite of PAF, in the guinea pig peritoneal cavity after i.p. administration of Escherichia coli LPS. Within 1 h of LPS administration, the level of PAF in the peritoneal fluid increased from 4.9 to 37.2 pmol/animal and decreased to the control value thereafter. In contrast, the level of lysoPAF gradually rose from 63.5 to 268 pmol/animal for up to 6 h. The activity of acetylhydrolase, which converts PAF to lysoPAF, in the peritoneal cavity increased in parallel with the increase in the lysoPAF level. The enzyme was distinguishable from phospholipase A2, because p-bromophenacylbromide (p-BPB), Ca2+, and ethylenediaminetetraacetic acid (EDTA) did not affect its enzymatic activity. In addition, this acetylhydrolase revealed similar biochemical properties to that detected in plasma. Both acetylhydrolases were resistant to trypsin treatment and had the same apparent molecular weight, as shown by gel-filtration column chromatography. These results suggest that the acetylhydrolase, which accumulates in the peritoneal cavity, infiltrates from the plasma in response to LPS, and then participates in the exclusion of PAF during endotoxin shock.