The central catecholamine neuron system in the mutant mouse tottering was examined by fluorescence histochemistry and biochemical analysis of catecholamine content. This single-locus neurological mutation expresses a reproducible alteration in central nervous system physiology characterized by spontaneous spike-wave and focal motor seizures in the absence of any previously recognized disturbance of cellular organization or brain size. Histochemical analysis showed a significant increase in the number of noradrenergic axons in terminal fields innervated by the nucleus locus ceruleus when compared with the wild type. A concomitant 100-200% rise in norepinephrine levels is found in the same areas, including hippocampus, cerebellum, and dorsal lateral geniculate. Catecholamine fibers and transmitter content in areas innervated by a second major noradrenergic system arising from the brainstem lateral tegmental neurons are unaltered. The terminal axons and transmitter content were both unchanged in nuclei receiving a dense dopaminergic innervation. Despite the hypertrophy of the locus ceruleus axonal plexus, the number and size of locus ceruleus cell somata were identical in both wild-type and tottering mice. These findings are consistent with a specific gene-linked alteration of developmental events controlling the number of axons produced by a single neuronal population in the mammalian brain.