In the substantia nigra of anesthetized and awake monkeys, presumptive dopamine cells of the pars compacta were electrophysiologically discriminated against non-dopaminergic cells of the pars reticulata by their lower discharge rate (0.5-8 vs 20-130 imp./s), their longer impulse duration (means 2.05 vs 0.92 ms), and their exclusive depression following systemic injection of the dopamine agonist apomorphine (24 out of 30 compacta neurons at 0.05-0.1 mg/kg s.c.).