The presence of neuromelanic pigment in neurons of the rat substantia nigra (SN) is a matter of controversy. The presence of neuromelanin in the SN of 15--24-month-old rats was investigated by electron microscopy (EM) and the findings compared to the morphology and distribution of neuromelanin in SN of dogs. In the rat, EM examination revealed a few small, evenly distributed neuromelanic granules in only 5% of nigral neurons. Dogs of 1 year of age showed a similar pattern; however, SN from adult and aged dogs exhibited abundant and larger neuromelanin granules which were also detectable by light microscopy. It is concluded that scanty neuromelanin is detectable by EM in rat SN and that the content of this pigment increases with age. The documented paucity of neuromelanin in rat SN neurons may explain why higher doses and longer treatment with MPTP is required in rats to obtain biochemical and behavioral signs of nigrostriatal dysfunction in this species.