Division of the axons of adrenergic neurones by crushing the postganglionic nerve trunks of rat superior cervical ganglia (SCG) at 6 days of age resulted in a permanent atrophy of the SCG reflected by a persistent decrease in the total protein content and in the activities of the enzymes tyrosine hydroxylase and DOPA decarboxylase. Administration of nerve growth factor (NGF) to rats with unilateral axotomy at a dose of 10 mug/g/day for the period 7-21 days of age resulted in hypertrophy of both normal and axotomised SCG. There was a progressive rise in the total protein content and in the activities of the two enzymes till the end of the treatment period in both SCG. After treatment ceased there was a progressive fall in the total protein content and activities of the two enzymes reaching a stable level after 4 weeks. The level reached for treated unoperated SCG remained elevated when compared to untreated control SCG. Axotomised treated SCG had approximately the same biochemical parameters as untreated control SCG and very much elevated over untreated axotomised SCG. These final levels persisted for at least 56 days after treatment had ceased. Animals showed a persistent ptosis after axotomy at 6 days of age but treatment with NGF resulted in a functional recovery by 11 weeks of age. It is suggested that there is normally a retrograde transfer of a factor durind development from the target cell to the perikarya of the neurone permitting survival if the appropriate connections are made. Failure to make such a contact results in cedd death. The cell death occurring normally, and the cell death resulting from axotomy, can both be prevented by NGF treatment leading to an hypertrophy of both SCG. This consistent with the hypothesis than NGF is the retrograde trophic agent for the sympathetic nervous system in the developing animal.