Human plasma β-endorphin (β-EP) and β-lipotropin (β-LPH) were measured simultaneously by radioimmunoassay in 4 women with regular menstrual cycle (4 follicular phase and 4 luteal phase), 5 normal men and 10 term fetuses. Mean plasma β-EP level in the totalsamplesof 8 women was 30.7 ± 5.8 pg/ml( ± SE), significantly (p < 0.03) higher than the β-EP level in men (12.2 ± 2.0 pg/ml). There was no significant sex difference in fetal plasma β-EP level. These data suggest that the existence of sex differences for β-EP secretion and release, and opioid peptides secretion and release mechanism might change with sexual maturation, due to changing sex steroids secretion.