The ontogeny of plasma beta-endorphin (B-E) and cortisol concentrations was studied in normal children (3-11 yr old) and adolescents (12-18 yr old) of several ethnic backgrounds. The morning plasma concentrations did not significantly differ as a function of sex or age for either B-E [males (n = 88), 7.19 +/- 0.39 (+/- SE) pmol/L; females (n = 73), 6.80 +/- 0.50 pmol/L] or cortisol [males (n = 87), 383 +/- 14 nmol/L; females (n = 69), 359 +/- 20 nmol/L]. There was a small but significant positive correlation between plasma B-E and cortisol concentrations. When sampled immediately after venipuncture, there were no significant differences in the plasma B-E or cortisol concentrations of children who cried during venipuncture compared with those who did not. These data suggest that morning plasma B-E and cortisol concentrations are constant throughout childhood, adolescence, and into early adulthood.