The three main postganglionic nerve branches of the superior cervical ganglion were divided on one side in 3--4-day-old rats. Five, 10, 20 and 60 days after the operation, the number of principal cells and small intensely fluorescent (SIF) cells, and the ganglion volume were estimated from a complete series of sections through each ganglion, in which catecholamines were histochemically demonstrated by formaldehyde-induced fluorescence. As compared with the contralateral, intact ganglion, the operated ganglion showed a rapid loss of principal cell bodies to about 1/20th of the control value, and the normally large postnatal increase in the volume of the ganglion failed to take place in the operated ganglion. Sham operation experiments showed that these changes were due to nerve division rather than disturbed blood supply and manipulation. The number of SIF cells increased in the intact ganglia from about 200 cells/ganglion at birth and reached the adult value, about 600 cells/ganglion by the 23rd postnatal day. An essentially similar postnatal increase in Sif cell number occurred in the experimental ganglia in spite of the marked loss of principal cells. The relative number of SIF cells increased from less than 1% of all cells (SIF cells and principal cells) in the control ganglia to over 10% in the operated ganglia, in which large aggregates of SIF cells formed 20 and 60 days after nerve division. It is concluded that different sets of developmental rules may apply to the SIF cells and the principal cells.