Two micrograms of beta-endorphin (beta-lipotropin61-91) injected intraventricularly in rats that had been treated with antiserum against somatostatin led to a 6- and 10-fold stimulation of the concentration of plasma growth hormone (somatotropin) measured 10 and 20 min after injection of the peptide, whereas 400 mug of methionine-enkephalin led to a 4- to 6-fold increase of levels of plasma growth hormone at 10 min with a rapid return to basal levels at later time intervals. At doses of 5 and 25 mug, beta-endorphin led to a 20- to 30-fold stimulation of levels of plasma growth hormone, the maximal effect being measured between 20 and 30 min after injection. These data suggest the possible role of the endogenous opiate-like peptides in the control of growth hormone secretion.