Summary Using a specific radioimmunoassay the concentrations of β-endorphin, β-lipotropin and corticotropin in maternal plasma were studied during pregnancy and in the early puerperium. Compared with the values in nonpregnant women, levels of β-endorphin and β-lipotropin were slightly lower in the first and second trimesters; corticotropin showed no difference in the first trimester. The concentrations of all these peptides increased towards term, most markedly in the case of corticotropin. The diurnal variation was studied near term and again in the early puerperium. β-Endorphin and corticotropin showed a parallel diurnal variation. The values in the morning and during the day were slightly higher during pregnancy than in the puerperium, whereas the values at midnight were similar. After vaginal delivery, a rapid and parallel decrease in the plasma levels of β-endorphin, β-lipotropin and corticotropin was found. In elective cesarean section, the values were highest 30 min after delivery, reflecting surgical stress, and then the decrease was similar to that after vaginal delivery. Thus parallel changes in the plasma levels of β-endorphin, β-lipotropin and corticotropin during pregnancy and in the early puerperium were found, which can be explained by their origin from a common precursor peptide, pro-opiomelanocortin. The rise in the plasma levels of corticotropin and endorphins near term and the exaggerated diurnal variation is probably a reflection of a maternal adaptation to stress.