We have previously reported that platelet-activating factor (PAF) is present in human amniotic fluid obtained from women in labor. We have also demonstrated that PAF, lyso-PAF, and alkyl acyl-sn-glycero-3-phosphocholine (AA-GPC) are present in human amnion tissue. In the reported study, we have investigated the enzymes involved in PAF metabolism in amnion tissue and their regulation. A phospholipase A2 activity has been demonstrated in amnion tissue which cleaves alkyl acyl (long-chain) sn-glycero-3-phosphocholine. The enzyme activity is not altered by Ca2+ and is distinctly different from the phospholipase A2 that we have previously characterized in this tissue. Amnion tissue contains acetyltransferase activity which requires Ca2+ and is associated with the microsomal fraction. Acetylhydrolase is also present in the cytosolic fraction of amnion tissue. Acetylhydrolase activity has also been demonstrated in amniotic fluid. The affinities of acetyltransferase (for lyso-PAF) and acetylhydrolase (for PAF) were unaffected by Ca2+. In the presence of Ca2+, however, the specific activity of acetyltransferase was increased four- to fivefold while that of acetylhydrolase was unaffected. Acetyltransferase and acetylhydrolase activities in fetal membranes and decidua were similar and were unchanged with gestational age. The possible role of PAF in the initiation of human parturition is discussed.