Gestational variations in maternal beta-endorphin (beta-end) secretion were investigated by simultaneous measurements of plasma and urinary immunoreactive beta-endorphin (ir-beta-end). Urinary ir-beta-end was extracted by using a Sep-Pak C18 column and was found to be stable for at least 24 hours at room temperature. Plasma and urine (2 hour collection) samples were obtained from 41 pregnant women and 7 non-pregnant women. Almost parallel increases in plasma and urinary ir-beta-end were observed during the course of pregnancy and there was a good correlation between them. Compared with non-pregnant values, significant increases (p less than 0.001) were observed after 36 weeks of gestation. We also studied chromatographic patterns of urinary extracts of non-pregnant and pregnant women. Both of them consist of 4 peaks and resemble each other. They were essentially similar to those of plasma extracts except for the third peak which appeared between the elution position of beta-lipotropin (beta-LPH) and beta-end. In conclusion, maternal beta-end secretions increase in late gestation not only in plasma but also in urine.