Tyrosine hydroxylase (TH) immunocytochemistry and quantitative light microscopic analysis of neurons in the substantia nigra pars compacta, the ventral tegmental area, and the locus coeruleus of weaver mutant mice at 12 and 15 weeks of age and unaffected controls revealed a decrease in the number of TH-positive neurons in the substantia nigra of weaver mice, but no change in cell numbers in the ventral tegmental area and locus coeruleus. Furthermore, the 15-week-old mutants showed a significant cell loss in the substantia nigra pars compacta compared with the 12-week-old group. Thus, the weaver mutant may provide a good model for studying the mechanisms of selective dopaminergic cell loss.