Endogenous opioids have been claimed to be responsible for pain threshold elevation in late pregnancy, by increased biosynthesis and/or reduced enzymatic degradation of opioid peptides in the CNS.CSF was obtained from 12 healthy women scheduled for elective cesarean section for CPD before anesthesia and from 8 healthy nonpregnant volunteers. Assays were performed of dynorphin-converting activity as well as levels of [Leu]enkephalin-Arg6, a derivative of prodynorphin-derived peptides.