We investigated the effects of nerve growth factor (NGF) and glucocorticoid on the differentiation of rat chromaffin cells during development (postnatal day 0, 5, 10, 15, 25) in vitro, by evaluating four parameters of cytological alterations; the percentage of process-bearing cells, the number of primary processes, the number of branches per cell and the total length of processes. Cultures of dissociated chromaffin cells from each developmental stage were maintained in medium containing a combination of NGF and/or the synthetic glucocorticoid, dexamethasone (DEX). All parameters for neuronal differentiation were highest in chromaffin cells from P0 animals when incubated in NGF(+)/DEX(-), but lowest in those from animals at P25 incubated in NGF(-)/DEX(+). In addition, chromaffin cells became progressively insensitive to the effects of NGF on cytological differentiation from P0 to P25. These results suggest that as development proceeds locally-derived factors become less potent on chromaffin cell differentiation.