Cell division in the rat superior cervical and stellate ganglia has been followed from 6 days before birth until 14 days after birth by [3H] thymidine autoradiography. Neuronal division ceased at the postnatal day 4 in superior cervical ganglia and postnatal day 3 in stellate ganglia, whereas division of non-neuronal cells continued throughout the entire period studied. Dividing cells in both ganglia were predominantly neuronal before birth and non-neuronal after the postnatal day 3. Nerve growth factor treatment resulted in an increase in the number of neurons in the ganglia when given either over the first four postnatal days while neuronal division was still occurring or from postnatal days 6--8 after neuronal division had ceased. The increase in neuron numbers was not due to increased neuronal division and was apparently due to an increased survival rate of differentiated neurons. An increase in the rate of division of non-neuronal cells accompained the administration of nerve growth factor.