A large proportion of neurons die during normal development of the nervous system via an active process known as apoptosis. We counted the total number of neurons and apoptotic neurons in the superior cervical ganglion of the GH Wistar rat strain, which possesses a neurotrophic deficit leading to excessive perinatal cell death, and in its normal counterpart (N) by using the optical disector method to quantify the extent of apoptosis during postnatal development. Total neuron numbers fell between postnatal days 3 and 14 by 10 and 40% in N and GH, respectively. In GH ganglia, 1.5% of neurons were apoptotic at any given time, as determined by the presence of condensed chromatin clumps. Some types of cell death have been associated with expression of the immediate-early genes c-fos and c-jun. Therefore, we used histological and immunocytochemical techniques to characterise individual neurons and to detect the products of these immediate-early genes during developmental cell death. All apoptotic cells were immunopositive for c-jun protein, whereas no c-jun protein was detected in nonapoptotic cells. Conversely, members of the fos family of transcription factors were detected in the nucleus of 60% of nonapoptotic cells but in only a minor proportion of cells undergoing apoptosis. These results indicate that c-jun occurs in neurons that are committed to die. This is the first situation in which the presence of jun protein has been correlated with normal programmed cell death in individual apoptotic neurons.