Regulation by hormones of nervous system development is well recognized in humans, laboratory animals and cultured nerve cells. Thyroxine (T4) and triiodothyronine (T3), the major thyroid hormones, act on brain development and maturation by binding to T3 nuclear receptors. T3 responsive genes have been identified with the T3 receptor as a superfamily of genes including cortisol and estrogens, necessary for adaptation and survival. Less defined are T3 and T4 actions on the peripheral nervous system. In chromaffin cells explanted from the adrenal of immature rats, T3 induces the enzyme tyrosine hydroxylase, involved in catecholamine synthesis. This action is similar but, so far, apparently independent from Nerve Growth Factor promotion of sympathetic and chromaffin cell growth. Mechanisms based on nuclear binding depend on multiple receptors functionally diversified; their selectivity of action over a wide range of early and late developmental patterns is an attractive hypothesis to be further explored.