This study evaluates the presence of proopiomelanocortin (POMC)-related peptides in four embryos and eight fetal pituitaries starting from 5 to 25 weeks of pregnancy. Moreover, fetal membranes (amnion and chorion) were also investigated. Freshly collected samples were boiled in acetic acid to destroy enzymes, homogenized and submitted to high performance liquid chromatography (linear gradient from 25 to 40% acetonitrile in 0.01 M HCI, in 15', 1.5 ml/min). The collected fractions were tested for the presence of beta-lipotrophin (beta-LPH), beta-endorphin(beta-EP), gamma-endorphin(gamma-EP) through RIAs. beta-EP and beta-LPH were detected from 7 weeks of pregnancy while gamma-EP appeared later. Only the cephalic portion of the embryos contained the peptides where beta-LPH predominates while no immunoreactivity was detected in the rostral one. In the fetal pituitary there is a progressive increase of gamma-EP according to the gestational age and both beta-EP and beta-LPH showed a trend toward constancy in the 15-25 week range. Amnion and chorion contain a significant amount of the three peptides. Their ontogenesis starts earlier than in the embryo; beta-LPH or beta-EP were detected at 5 weeks of pregnancy. In both tissues beta-EP was higher in the first than in the second trimester. These data demonstrate a different pattern of POMC ontogeny and processing between the conceptus and his environment. This suggests that the POMC-related opiod system of the fetus and of fetal adnexes are independent of each other, possibly subserving to different functions.