The opioid peptide, beta-endorphin, originates from proopiomelanocortin (POMC) under the influence of corticotropin releasing hormone (CHR). It increases the threshold of pain and has a certain influence on the formation of hypophyseal hormones, especially in stress. It is found that beta-endorphin stimulates the secretion of prolactin, a growth hormone, and vasopressin; it inhibates formation of follicle-stimulating and luteinizating hormones, oxytocin and dopamine, and gonadotropin, a releasing hormone. The process of acetylization decreases its activity. The results of experimental trials revealed that acetylisation in the foetal period was absent. The aim of the study was to define beta-endorphin concentration during normal vaginal labor and Cesarean section. Samples of peripheral blood of patients with spontaneous vaginal labor (n = 15) and of those in whom labor was operatively terminated (Cesarean section) (n = 10), were analysed. Values of this opiate were determined in the umbilical cord of newborn infants, in the amniotic fluid and placental compartment. The obtained results were statistically analysed. In intrapartum beta-endorphins were significantly increased reaching the highest level during expulsion (326 pg/ml); in the placental compartment these values were higher (in retroplacental blood 514 pg/ml) reaching the highest value of 917 pg/ml, p less than 0.01 in the placenta. In Cesarean section beta-endorphin values in the peripheral blood showed no significant differences during spontaneous vaginal labor. However, increased values of this natural opiate were observed six hours after surgery. Beta-endorphin concentrations in the placental compartment and the placenta during normal vaginal labor were significantly higher in comparison with labor by Cesarean section (p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)