The effect of gamma-and beta-endorphins, leu- and met-enkephalins on the growth and differentiation of nervous tissue was studied in organotypic cultures of the sympathetic ganglia of newborn rats. The growth of explants in living cultures and preparations stained by the Holmes method was analyzed. It was established that endorphins are capable of stimulating the growth of neurites from the explant, of increasing the number of glial and fibroblastoid cells in the zone of the growth. The mean value of the maximal magnitude of the zone of the growth in normalcy was 464 +/- 136 micron, that on addition of leu- and met-enkephalins, gamma- and beta-endorphins 879 +/- 161, 900 +/- 160, 959 +/- 170, and 1056 +/- 137 microns. The growth effect induced by endorphins was demonstrated within a wide dose range--from 10(-7) to 10(-14) M. Naloxone did not inhibit the stimulant action of the peptides. It is suggested that opioid neuropeptides can be used as a source of nonspecific growth factors for nervous tissue.